
<PAGE>
 
                                                                   Exhibit 10.30





              NOTE AND MORTGAGE MODIFICATION AND SPREADER AGREEMENT



                   NOTE AND MORTGAGE MODIFICATION AND SPREADER AGREEMENT
         (this "Agreement"), dated as of November __, 1997, between BOS-
                ---------
         TON PROPERTIES LIMITED PARTNERSHIP ("Mortgagor"), a Delaware
                                              ---------
         limited partnership, having an address at 599 Lexington Avenue,
         New York, New York 10022, and JOHN HANCOCK MUTUAL LIFE INSUR-
         ANCE COMPANY ("Mortgagee"), a Massachusetts corporation, having
                        ---------
         its principal office at John Hancock Place, Post Office Box
         111, Boston, Massachusetts 02117, Attention:  Real Estate In-
         vestment Group, T-52.


                                   WITNESSETH:

                   WHEREAS, Mortgagor (a) is, on the date of delivery
         hereof, the owner of fee title to a parcel of land and the im-
         provements thereon known as 875 Third Avenue, New York, New
         York and the easements appurtenant thereto (collectively, the
         "Original Fee Parcel"), all as more particularly described in
          -------------------
         Exhibit A-1 hereto, and (b) shall, prior to the recording of
         -----------
         this Agreement (as more fully described below), be the owner of
         fee title to the Corner Parcel and all Improvements thereon (as
         such terms are defined in the Mortgage, as hereinafter de-
         fined), which Corner Parcel is contiguous to the Original Fee
         Parcel and is more particularly described on Exhibit A-2 hereto
         (the Original Fee Parcel and the Corner Parcel as collectively
         described on Exhibit A-3 hereto are herein collectively re-
                      -----------
         ferred to as the "Premises");
                           --------

                   WHEREAS, Mortgagee is on the date hereof the owner
         and holder of the mortgages described in Exhibit B hereto,
                                                  ---------
         which mortgages were consolidated, extended and modified pursu-
         ant to that certain Consolidation, Extension and Modification
         Agreement (the "Consolidation Agreement"), dated as of May 11,
                         -----------------------
         1988, between Kenvic Associates, a New York general partnership
         ("Original Mortgagor") and Mortgagee, recorded on May 18, 1988
           ------------------
         in the Office of the City Register, New York County (the
         "Register's Office") in Reel 1403, Page 1793, so as to form a
          -----------------
         single first lien on the Mortgaged Premises (as such term is
         hereinafter defined) in the original principal amount of
         $180,000,000, and were further modified (i) pursuant to that
         certain Modification Agreement, dated as of May 30, 1990, be-
         tween Original Mortgagor and Mortgagee, recorded on June 28,
         1990 in the Register's Office in Reel 1705, Page 1760 (the
         "1990 Modification Agreement"), (ii) pursuant to that certain
          ---------------------------
         Note and Mortgage Modification Agreement, dated as of July 23,
         1992, between Original Mortgagor and Mortgagee, recorded on
<PAGE>
 
         July 30, 1992 in the Register's Office in Reel 1892, Page 434
         (the "1992 Modification Agreement"), and (iii) pursuant to that
               ---------------------------
         certain Note and Mortgage Modification and Spreader Agreement,
         dated as of December 29, 1995, between Original Mortgagor and
         Mortgagee, recorded on January 5, 1996 in the Register's Office
         in Reel 2279, Page 443 (the "1995 Modification Agreement");
                                      ---------------------------

                   WHEREAS, the Mortgage (as such term is hereinafter
         defined) secures, among other things, the payment of the prin-
         cipal of and premium, if any, and interest on the Notes (as
         such term is hereinafter defined) in accordance with the terms
         of the Notes and the Mortgage;

                   WHEREAS, as of the date hereof, (i) Original Mort-
         gagor has, in exercise of its rights under Section 6 of the
         1995 Modification Agreement, conveyed to Mortgagor all of
         Original Mortgagor's right, title and interest in and to the
         Original Fee Parcel, (ii) Mortgagee and Original Mortgagor have
         terminated the Corner Parcel Lease pursuant to that certain
         Termination of Lease, dated as of the date hereof, (iii) Mort-
         gagee has conveyed to Mortgagor Mortgagee's fee title in and to
         the Corner Parcel, and (iv) Mortgagor has agreed to assume all
         of Original Mortgagor's obligations under the Notes, the Mort-
         gage and the other Loan Documents pursuant to the Assumption
         (as such terms are hereinafter defined), including, without
         limitation, the obligations of Original Mortgagor to make pay-
         ments of principal and interest under and pursuant to the Notes
         (the transactions described in subparagraphs (i), (ii), (iii)
         and (iv) above shall hereinafter be collectively referred to as
         the "Transfer"; all documents and instruments evidencing, cre-
              --------
         ating or otherwise relating to or executed in connection with
         the Transfer shall hereinafter be collectively referred to as
         the "Transfer Documents");
              ------------------

                   WHEREAS, Mortgagor and Mortgagee desire to spread the
         lien of the Mortgage to cover all of the Premises, including,
         without limitation, both the Original Fee Parcel and Mort-
         gagor's fee title to the Corner Parcel (including the Improve-
         ments thereon); and

                   WHEREAS, Mortgagor and Mortgagee desire to further
         modify certain terms and conditions of the Mortgage in the man-
         ner set forth herein.

                   NOW, THEREFORE, for good and valuable consideration,
         the receipt and sufficiency of which are hereby acknowledged,
         Mortgagor and Mortgagee hereby agree as follows:

                                      -2-
<PAGE>
 
                   1.  Definitions.
                       -----------

                   (a)  Any capitalized term used in this Agreement and
         not defined herein shall have the respective meaning assigned
         to such term in the Notes or in the Mortgage.  Each term de-
         fined below, when used in this Agreement, in the Mortgage, the
         Notes or in any of the other Loan Documents, shall have its
         respective meaning set forth below.

                   (b)  The term "1995 Amendment" shall mean the Amend-
                                  --------------
         ment Agreement, dated as of December 29, 1995, between Original
         Mortgagor and Mortgagee, and recorded in the Register's Office
         at Reel 2279, Page 516.

                   (c)  The term "Assignment of Leases" shall mean the
                                  --------------------
         Assignment of Leases, dated as of May 11, 1988, between Origi-
         nal Mortgagor and Mortgagee, recorded in the Register's Office
         on May 18, 1988 in Reel 1403, Page 1899, as amended by the 1995
         Amendment, and as further amended by that certain Amendment
         Agreement (the "1997 Amendment"), dated as of the date hereof,
                         --------------
         between Mortgagor and Mortgagee, and intended to be recorded in
         the Register's Office following the execution thereof, as may
         be further amended, modified or supplemented from time to time.

                   (d)  The term "Assignment of Rents" shall mean the
                                  -------------------
         Assignment of Rents, dated as of May 11, 1988, between Original
         Mortgagor and Mortgagee, recorded in the Register's Office on
         May 18, 1988 in Reel 1403, Page 1886, as amended by the 1995
         Amendment, and as amended further by the 1997 Amendment, as may
         be further amended, modified or supplemented from time to time.

                   (e)  The term "Assumption" shall mean the Assumption,
                                  ----------
         dated as of the date hereof, by Mortgagor for the benefit of
         Mortgagee and Original Mortgagor and their respective succes-
         sors and assigns.

                   (f)  The term "Consent" shall mean the Consent, dated
                                  -------
         as of the date hereof, by Mortgagee for the benefit of Original
         Mortgagor and Mortgagor and their respective successors and
         assigns.

                   (g)  The term "Controlling Party" shall mean the
                                  -----------------
         Mortgagor General Partner and any other Person or Persons,
         whether directly or indirectly, exercising control over the
         business, management or operations of, or having the right to
         make, direct or veto significant business decisions for, Mort-
         gagor, including (without limitation) through ownership or ac-
         quisition of equity interests or other beneficial interests in

                                      -3-
<PAGE>
 
         Mortgagor, through the exercise of rights contained in the Gov-
         erning Documents of Mortgagor, through the amendment or mod-
         ification of the Governing Documents of Mortgagor, or otherwise
         by contract or agreement, provided that a Person shall not be
                                   --------
         deemed to be a Controlling Party solely by virtue of its acting
         as manager or managing agent of the Mortgaged Premises.

                   (h)  The term "Corner Parcel Indemnity" shall mean
                                  -----------------------
         the Indemnification Agreement, dated as of the date hereof,
         between Mortgagor and Mortgagee, as amended, modified or
         supplemented from time to time, which agreement has been at-
         tached hereto as Exhibit C and shall be deemed to be incorpo-
                          ---------
         rated in, and made a part of, the Mortgage.

                   (i)  The term "Environmental Indemnity" shall mean
                                  -----------------------
         the Environmental Indemnity and Agreement, dated as of the date
         hereof, between Mortgagor and Mortgagee, as amended, modified
         or supplemented from time to time, which agreement has been
         attached hereto as Exhibit D and shall be deemed to be incorpo-
                            ---------
         rated in, and made a part of, the Mortgage.

                   (j)  The term "Loan Documents" shall mean, collec-
                                  --------------
         tively, the Notes, the Consolidation Agreement, the Mortgage,
         the 1990 Modification Agreement, the 1992 Modification Agree-
         ment, the 1995 Modification Agreement, this Agreement, the Cash
         Collateral Agreement, the Environmental Indemnity, the Corner
         Parcel Indemnity, the Assignment of Rents, the Assignment of
         Leases, the Tax Processing Agreement, and any other documents,
         instruments or agreements entered into pursuant to any such
         documents, or in connection therewith.

                   (k)  The term "Mortgagor General Partner" shall mean
                                  -------------------------
         Boston Properties, Inc., a Delaware corporation.

                   (l)  The term "1997 Modification Documents" shall
                                  ---------------------------
         mean, collectively, this Agreement, the 1997 Amendment, the
         Environmental Indemnity, the Corner Parcel Indemnity, the As-
         sumption, the Consent, the Release, the Transfer Documents and
         all other documents, instruments or agreements entered into
         pursuant thereto or in connection therewith.

                   (m)  The term "Release" shall mean the Mutual Waiver
                                  -------
         and Release Agreement, dated as of the date hereof, between
         Mortgagee and Original Mortgagor, as amended, modified, or
         supplemented from time to time.

                   (n)  The term "Tax Processing Agreement" shall mean
                                  ------------------------
         the Tax Processing Agreement, dated as of May 10, 1988, between

                                      -4-
<PAGE>
 
         Original Mortgagor and Mortgagee, as amended by the 1995 Amend-
         ment and 1997 Amendment, and as may be further amended, modi-
         fied or supplemented from time to time.

                   (o)  The term "Title Commitment" shall mean the Com-
                                  ----------------
         mitment for Title Insurance (Title No. TS-109932) relating to
         the Mortgaged Premises, dated as of July 1, 1997, issued by
         TitleServ Agency of New York City, Inc., as may be amended,
         modified and supplemented from time to time.

                   2.  Acknowledgment by Mortgagor.  Mortgagor acknowl-
                       ---------------------------
         edges and confirms that the Transfer was effected subject to
         the lien and security interest of the Mortgage, the Assignment
         of Rents, the Assignment of Leases, the Tax Processing Agree-
         ment, the Cash Collateral Agreement and the other Loan Docu-
         ments, and that Mortgagor pursuant to the Assumption has as-
         sumed the obligations of Original Mortgagor under the Notes,
         the Mortgage, the Assignment of Rents, the Assignment of
         Leases, the Tax Processing Agreement, the Cash Collateral
         Agreement, and the other Loan Documents.  From and after the
         date hereof, the term "Mortgagor", "Assignor", "Obligor",
         "Maker" or such other term as used in the Mortgage, the Notes,
         the Assignment of Rents, the Assignment of Leases, the Tax Pro-
         cessing Agreement, the Cash Collateral Agreement, and the other
         Loan Documents shall mean Boston Properties Limited Partnership
         and its permitted successors and/or assigns.

                   3.  Mortgage to Be Spread.
                       ---------------------

                   (a)  The term "Land", as defined and used in the
                                  ----
         Mortgage, shall be amended to include (without limitation) the
         Corner Parcel described on Exhibit A-2 hereto.
                                    -----------

                   (b)  The term "Mortgaged Premises", as defined and
                                  ------------------
         used in the Mortgage, shall be amended to include, without lim-
         itation, Mortgagor's fee interest in the Corner Parcel, includ-
         ing (without limitation) the Improvements thereon and the Space
         Leases applicable to the Corner Parcel and the Improvements
         thereon.

                   (c)  The Mortgage and the lien thereof is hereby
         spread to cover Mortgagor's fee interest in the Corner Parcel,
         including (without limitation) Mortgagor's right, title and
         interest in and to the Improvements thereon and the Space
         Leases applicable to the Corner Parcel and the Improvements
         thereon.

                   (d)  Section 5.01(f) is hereby deleted from the Mort-
         gage in its entirety.

                                      -5-
<PAGE>
 
                   (e)  Section 4(e) of the 1995 Modification Agreement
         is hereby deleted in its entirety.

                   4.  Sales, Transfers, etc.  Section 5.01(n) of the
                       ---------------------
         Mortgage is hereby amended and restated, in its entirety, as
         follows:

                        "(n) if Mortgagor or any of its successors or
              permitted assigns shall, voluntarily or by operation of
              law, directly or indirectly (including, without limita-
              tion, by merger, consolidation or transfer, conveyance or
              other disposition of over 49% of the ownership interests
              in Mortgagor or by merger, consolidation or transfer, con-
              veyance or other disposition of over 49% of the ownership
              interests of any Controlling Party), sell, assign, trans-
              fer, convey or otherwise dispose of all or any part of the
              Mortgaged Premises, unless (i) at least 30 days prior to
                                          -
              such sale, transfer, conveyance or disposition, Mortgagor
              shall notify Mortgagee of such transaction, (ii) Mortgagee
                                                           --
              shall consent thereto in writing, which consent may be
              withheld by Mortgagee in its sole and absolute discretion,
              (iii) subject to the provisions of Section 5.02 hereof,
               ---
              the Mortgagor executing this Mortgage shall remain
              primarily liable for the indebtedness secured hereby and
              the performance of all the terms, covenants and conditions
              of this Mortgage, (iv) Mortgagee is furnished by Mortgagor
                                 --
              with true, correct and complete copies of the documenta-
              tion effecting such transfer or encumbrance within 5 days
              after such transaction, and (v) the transferee(s) exe-
                                           -
              cute(s) this Mortgage as a security agreement and such
              financing statements and other documents, certificates and
              instruments as may be required by Mortgagee, so that Mort-
              gagee will have a valid and perfected security interest in
              all personal property acquired by any successors and
              assigns of Mortgagor for use in or upon such conveyed
              property and delivers all such documents, certificates and
              instruments to Mortgagee within 5 days after such sale,
              conveyance, transfer or disposition.  Notwithstanding the
              foregoing, to the extent equity interests in Mortgagor or
              any Controlling Party are (i) freely traded on a
              nationally recognized public market or exchange (a "Public
                                                                  ------
              Market"), and (ii) have been registered for sale with the
              ------
              Securities and Exchange Commission and are otherwise
              subject to filing and registration requirements under the
              Securities Exchange Act of 1934, as amended, the buying
              and selling of such equity interests on such Public Market
              in the ordinary course shall not require Mortgagee's
              consent hereunder, provided that any such transaction
              shall not result in a change (directly or indirectly) of
              the Person or Persons exercising control over

                                      -6-
<PAGE>
 
              the business, management or operations of, or having the
              right to make, direct or veto significant business deci-
              sions for, Mortgagor or such Controlling Party."

                   5.  Exercise of One Time Assumption.  Original Mort-
                       -------------------------------
         gagor has exercised its rights under Section 6 of the 1995 Mod-
         ification Agreement to "Transfer" (as such term is defined
         therein) all of the Mortgaged Premises to Mortgagor, and as
         such rights of "Transfer" under said section were limited to a
         one time occurrence, said Section 6 of the 1995 Modification
         Agreement is hereby deleted in its entirety.

                   6.  Liability of Mortgagor.
                       ----------------------

                   (a)  In any action brought at law or in equity to
         enforce the obligations of Mortgagor to pay any indebtedness or
         to perform any other monetary obligation or any non-monetary
         obligation created or arising under the Consolidation Agree-
         ment, the Mortgage, the Notes, the Assignment of Leases, the
         Assignment of Rents, the Cash Collateral Agreement, or under
         any of the other Loan Documents, the judgment or decree shall
         be enforceable against Mortgagor only to the extent of its in-
         terest in the Mortgaged Premises (or the property subject to
         such other instrument or agreement described above), and Mort-
         gagee shall not seek or claim recourse against any other assets
         of Mortgagor or against any party holding partnership interests
         or other ownership interests, equitable interests or other ben-
         eficial interests, whether direct or indirect, in Mortgagor
         (each, a "Constituent Party" and collectively, "Constituent
                   -----------------                     ----------- 
         Parties"), or any principal, member, officer, director, share-
         -------
         holder or equity owner of any such Constituent Party and any
         such judgment shall not be subject to execution on, nor be a
         lien on, assets of Mortgagor (or of its Constituent Parties or
         any principal, member, officer, director, shareholder or other
         equity owners of its Constituent Parties) other than the Mort-
         gaged Premises (or the property subject to such other instru-
         ment or agreement described above), provided that the foregoing
                                             --------
         shall not relieve Mortgagor or any other Controlling Party of
         any liability for, nor prejudice or limit the rights of Mort-
         gagee by reason of, any of the following, to the extent of any
         damage arising directly or indirectly to Mortgagee:

                   (i)   fraud or misrepresentation of Mortgagor, and
                         physical waste of the Mortgaged Premises aris-
                         ing from the gross negligence or wilful miscon-
                         duct of Mortgagor;

                   (ii)  any rents, issues or profits collected more
                         than one (1) month in advance of their due
                         dates;

                                      -7-
<PAGE>
 
                   (iii) any Misapplication (as hereinafter defined) of
                         loan proceeds, rents, issues or profits, secu-
                         rity deposits, any other payments from tenants
                         or occupants (including without limitation,
                         lease termination fees), insurance proceeds or
                         condemnation awards;

                   (iv)  liability under the Environmental Indemnity or
                         the environmental covenants, environmental con-
                         ditions and environmental indemnifications con-
                         tained in the Mortgage; 

                   (v)   liability under the Corner Parcel Indemnity;

                   (vi)  personalty or fixtures which Mortgagor, di-
                         rectly or through an agent, removed from the
                         Mortgaged Premises and which were not replaced
                         by items of equal or greater utility than the
                         personalty or fixtures so removed, unless such
                         personalty or fixtures so removed are unneces-
                         sary to properly operate the Mortgaged Premises
                         or are obsolete, provided that, for purposes of
                         this subparagraph (vi), neither a receiver for
                         the Mortgaged Premises appointed solely by or
                         at the request of Mortgagee, an agent appointed
                         solely by Mortgagee, nor the Mortgagee acting
                         as mortgagee-in-possession, shall be deemed to
                         be an agent of Mortgagor;

                   (vii) any amount equal to the sum of all payments
                         made by Mortgagor to holders of subordinate
                         mortgages (not consented to in writing by Mort-
                         gagee) on the Mortgaged Premises during any pe-
                         riod in which Mortgagor is in default of pay-
                         ment of principal or interest under the Notes
                         or of any escrow payments required under the
                         Mortgage in respect of real property taxes or
                         assessments; or

                  (viii) reasonable attorney's fees, court costs and
                         other expenses incurred by Mortgagee in connec-
                         tion with the successful enforcement of Mort-
                         gagor's personal liability as set forth herein.

                   (b)  As used in this Section 6, the term "Misapplica-
         tion" shall mean (1) the distribution or payment of cash flow
         by Mortgagor to, or for the benefit of, any holder of a mort-
         gage on the Mortgaged Premises subordinate to the Mortgage,
         which mortgage has not been consented to in writing by Mort-
         gagee (a "Distribution"), at such time as Mortgagor has failed
                   ------------

                                      -8-
<PAGE>
 
         to pay or caused to be paid any of the following:  (A) the
         principal and interest then due under the Notes and Mortgage,
         (B) any other amounts then due under the Notes, the Mortgage,
         the Cash Collateral Agreement, or any other Loan Documents, or
         (C) any amounts then due in connection with the operation, man-
         agement, maintenance and repair of the Mortgaged Premises, in-
         cluding (without limitation) amounts then due for tenant im-
         provements or other capital expenditures, real property taxes
         or assessments (provided, however, Mortgagor shall be deemed to
         have paid real property taxes to the extent of payments made by
         Mortgagor into escrow pursuant to the Tax Processing Agreement)
         insurance premiums, salaries, service contracts, utilities, or
         labor or material for work performed at or for the benefit of
         the Mortgaged Premises, or (2) the use by Mortgagor of security
         deposits, insurance proceeds, condemnation awards or other re-
         stricted funds for purposes other than the purposes set forth
         in any lease, contract or other agreement binding upon Mort-
         gagor and governing the use of such funds.  Notwithstanding the
         foregoing, the following circumstances alone shall not consti-
         tute a "Misapplication":  if Mortgagor shall pay a Distribu-
         tion, at such time as there shall be no outstanding Event of
         Default, and amounts are then due and owing by Mortgagor either
         (1) for capital improvement costs, tenant improvement costs or
         other items for which Mortgagor has submitted a Withdrawal Re-
         quest (as defined in the Cash Collateral Agreement) and for
         which Mortgagor is then entitled to receive amounts under the
         Cash Collateral Agreement (provided that Mortgagor shall
         promptly pay such expenditures upon receipt of such funds from
         the Cash Collateral Account) or (2) for expenditures incurred
         for materials (the cost of which is reimbursable under the Cash
         Collateral Agreement) purchased or in fabrication and to be
         delivered pursuant to binding contracts reasonably committed to
         at the time of execution taking into account sums on deposit in
         the Cash Collateral Account.

                   (c)  In Section 5.02 of the Mortgage, (i) the words
         "Limitation on Mortgagor's Liability" are hereby deleted from
         the caption of such Section 5.02, and (ii) the entire second
         sentence of such Section 5.02 is hereby deleted.

                   (d)  This Section 6 as it applies to Mortgagor shall
         supersede, amend and restate in its entirety Section 7 of the
         1995 Modification Agreement.

                                      -9-
<PAGE>
 
                   7.  Certain Additional Modifications.
                       --------------------------------

                   (a)  Section 2.09.7 of the Mortgage, as amended by the 1995
         Modification Agreement, is hereby amended by substituting "Mortgagor
         General Partner" for "Managing General Partner of Mortgagor", and by
         deleting the text: "(or, by the managing member of Mortgagor if
         Mortgagor has converted to a limited liability company in accordance
         with the terms of the Mortgage)".

                   (b)  Section 2.19 of the Mortgage, as amended by the 1995
         Modification Agreement, is hereby amended by deleting the last sentence
         therefrom in its entirety.

                   (c)  Section 2.20 of the Mortgage, as amended by the 1995
         Modification Agreement, is hereby amended by substituting "Mortgagor"
         for "a Permitted Controlling Party".

                   (d)  Section 2.21 of the Mortgage, as amended by the 1995
         Modification Agreement, is hereby amended by substituting "the
         Mortgagor General Partner" for "a Permitted Controlling Party".

                   (e)  Section 2.22 of the Mortgage, as amended by the 1995
         Modification Agreement, is hereby amended and restated in its entirety,
         as follows:

                        "2.22.  Existence.  Mortgagor shall preserve and 
                                ---------
              maintain its existence as a limited partnership under the laws of
              the State in which it is organized and its rights, franchises,
              licenses and privileges material to its business."

                   (f)  Section 4.04 of the Mortgage, as amended by the 1995
         Modification Agreement, is hereby amended by substituting "Mortgagor
         General Partner" for "Managing General Partner of Mortgagor", and by
         deleting the text: "(or, by the managing member of Mortgagor if
         Mortgagor has converted to a limited liability company in accordance
         with the terms of the Mortgage)".

                   (g)  Sections 5.01(h) and 5.01(i) of the Mortgage are hereby
         amended (i) by substituting "Controlling Party" for "general partner"
         in the first, twenty-first and twenty-third lines of Section 5.01(h)
         and in the tenth, twelfth and sixteenth lines of Section 5.01(i), and
         (ii) by inserting in the second line of Section 5.01(i) after
         "proceeding against Mortgagor", the text: ", any Controlling Party".

                                      -10-
<PAGE>
 
                   (h)  Section 7.04 of the Mortgage is hereby amended and
         restated, in its entirety, to read as follows:

                        "7.04.  Notices, etc.  All notices, demands, requests,
                                ------------
              consents, approvals and other instruments under this Mortgage or
              the Notes shall be in writing and shall be deemed to have been
              actually or properly given if and when mailed by first-class
              registered or certified mail, return receipt requested, postage
              prepaid, or by personal delivery (with a receipt obtained
              therefor) or recognized overnight courier service, addressed (a)
                                                                            -
              if to Mortgagor, to Boston Properties Limited Partnership, 599
              Lexington Avenue, New York, New York 10022, Attention, Robert
              Selsam, or at such other address as Mortgagor may have designated
              by notice to Mortgagee, with a copy to Goodwin, Proctor & Hoar,
              LLP, Exchange Place, Boston, Massachusetts 02109, Attention,
              Edward M. Schulman, Esq., with a copy to Weil, Gotshal & Manages
              LLP, 767 Fifth Avenue, New York, New York 10153, Attention, Alan
              J. Pomerantz, Esq. or to such other firms as Mortgagor may direct
              in writing, (b) if to Mortgagee originally named herein, to John
                           -
              Hancock Mutual Life Insurance Company, John Hancock Place, P.O.
              Box 111, Boston, Massachusetts 02117, Attention, Real Estate
                                                    ---------
              Investment Group, Floor T-52, or at such other address as
              Mortgagee may have designated by notice to Mortgagor, or (c) if
                                                                        -
              to any holder of any Note, other than Mortgagee originally named
              herein, at such address as such holder may have designated by
              notice to Mortgagor, or, until an address is so designated, to and
              at the address of the last holder of such Note so designating an
              address to Mortgagor. The foregoing insertion of Mortgagor's
              mailing address shall be deemed to be a request by Mortgagor that
              a copy of any notice of default and of any notice of sale
              hereunder be mailed to Mortgagor at such address as provided by
              law."

                   (i)  Mortgagor and Mortgagee covenant and agree that all
         representations, warranties, covenants, liabilities and obligations of
         Mortgagor under the Corner Parcel Indemnity (attached as Exhibit C
                                                                  ---------
         hereto), the Environmental Indemnity (attached as Exhibit D hereto)
                                                           ---------
         and under any of the other 1997 Modification Documents and under the
         Transfer Documents shall be deemed to be incorporated in the Mortgage
         and shall be secured by the lien of the Mortgage and by any lien
         created by, or arising under, any other Loan Document.

                                      -11-
<PAGE>
 
                   8.  Cross-Defaults. Mortgagor shall comply in all respects
                       --------------
         with Mortgagor's covenants and obligations under this Agreement, the
         Cash Collateral Agreement, the Environmental Indemnity, the Corner
         Parcel Indemnity, the Assignment of Rents, the Assignment of Leases,
         and all of the other Loan Documents, and any default by Mortgagor in
         the performance and observance of its covenants and obligations under
         any of such documents shall (subject to any notice and cure provisions
         specifically set forth in such documents, or, if none are set forth in
         such documents, subject to the notice and cure provisions contained in
         Section 5.01(d) of the Mortgage) be deemed to be an Event of Default by
         Mortgagor under the Mortgage.

                   9.  Modification Agreement Governs. In the event of any
                       ------------------------------
         conflict or inconsistency between this Agreement and the terms and
         conditions of the Mortgage, the Notes, or any other Loan Document, then
         the terms and conditions of this Agreement shall govern and prevail,
         and the conflicting or inconsistent terms and conditions of the
         Mortgage, the Notes or such other Loan Document shall be deemed to be
         amended accordingly.

                   10.  References to "Mortgage" and "Notes". From and after the
                        ------------------------------------
         date hereof, (a) all references herein, in the Consolidation
         Agreement, the Mortgage, the Notes or in any other Loan Document to
         "the Mortgage" and to "this Mortgage" shall be deemed to refer to the
         Consolidation Agreement as amended by the 1990 Modification Agreement,
         the 1992 Modification Agreement, the 1995 Modification Agreement, and
         by this Agreement, and as hereafter amended, modified or supplemented
         from time to time, and (b) all references herein and in the
         Consolidation Agreement, the Mortgage, the Notes or in any other Loan
         Document to "the Notes" or "this Note" shall be deemed to refer to the
         Notes, as amended by the 1990 Modification Agreement, the 1992
         Modification Agreement, the 1995 Modification Agreement, and by this
         Agreement, and as hereafter amended, modified or supplemented from time
         to time.

                   11.  No Offsets or Defenses. Mortgagor hereby represents and
                        ----------------------
         warrants to Mortgagee that, as of the date hereof, (a) the outstanding
         principal amount owing under the Notes and secured by the Mortgage is
         One Hundred Eighty Million and No/100 Dollars ($180,000,000.00), which
         amount Mortgagor covenants to pay in accordance with all of the terms
         and provisions of the Notes and the Mortgage; and (b) Mortgagor has no
         offsets or defenses to the Mortgage, the Notes or any of the Loan
         Documents of any kind whatsoever (and Mortgagor hereby expressly waives
         any and all of the same).

                                      -12-
<PAGE>
 
                   12.  Representations and Warranties. Mortgagor hereby
                        ------------------------------
         represents and warrants to Mortgagee as follows:

                   (a)  This Agreement, the other 1997 Modification Documents
         and the Transfer Documents have been duly executed by the Mortgagor and
         each constitutes the legal, valid and binding obligation of Mortgagor,
         enforceable against Mortgagor in accordance with its terms, except as
         limited by applicable bankruptcy, insolvency, moratorium,
         reorganization and similar laws affecting the rights of creditors
         generally, and except as limited by equitable principles of general
         application.

                   (b)  No authorization, consent, approval, license or formal
         exemption from, nor any filing, declaration or registration with, any
         federal, state, local or foreign court, governmental agency or
         regulatory authority or any other person or entity is required in
         connection with the making and performance by Mortgagor of this
         Agreement, the other 1997 Modification Documents or the Transfer
         Documents, except for such consents, approvals and authorizations
         which have already been obtained.

                   (c)  Mortgagor is a limited partnership duly organized,
         validly existing and in good standing under the laws of the State of
         Delaware and has all requisite power and authority to own and operate
         its properties, to carry on its business as now conducted and proposed
         to be conducted, to execute, deliver and perform this Agreement, the
         other 1997 Modification Documents and the Transfer Documents. This
         Agreement has been duly authorized by all necessary action on the part
         of Mortgagor.

                   (d)  The execution, delivery, recordation and performance of
         this Agreement, the other 1997 Modification Documents and the Transfer
         Documents and the consummation of the transactions contemplated hereby
         and thereby will not contravene, result in any violation of, result in
         a breach of, be in conflict with or constitute a default or event of
         default under any term or provision of Mortgagor's partnership
         agreement or of any term or condition of any contract, agreement, lease
         or instrument, judgment, decree, order, statute, rule, regulation,
         ordinance, franchise, certificate, permit or the like applicable to
         Mortgagor or the Mortgaged Premises or any part thereof or by which
         Mortgagor or its properties or assets may be bound or affected.

                   (e)  To the best of Mortgagor's knowledge, no consent,
         approval, order or authorization of, or designation, registration,
         declaration or filing with, or notice to or action to be taken in
         respect of any governmental or public authority,

                                      -13-
<PAGE>
 
         body or agency is required in connection with the valid execution,
         delivery and performance by Mortgagor of this Agreement, the other 1997
         Modification Documents or the Transfer Documents or the carrying out of
         any of the transactions contemplated hereby or thereby except the
         recordation of this Agreement, the 1997 Amendment, the other applicable
         1997 Modification Documents and the applicable Transfer Documents with
         the Register's Office and the filing of all necessary financing
         statements with respect thereto.

                   (f)  To the best of Mortgagor's knowledge, there is no
         action, suit, proceeding or investigation pending or threatened, or
         any basis therefor known to Mortgagor which questions the validity of
         the Mortgage, the Notes, any other Loan Document, the Consolidation
         Agreement, this Agreement, the 1997 Modification Documents or the
         Transfer Documents, or any action taken or to be taken pursuant
         thereto.
                   (g)  To the best of Mortgagor's knowledge, except as
         previously disclosed in writing to the Mortgagee, (A) there are no
         pending or anticipated material modifications to any Space Leases, (B)
         except for the Consent and that certain letter agreement dated November
         ____, 1997 among Mortgagee, Mortgagor and Original Mortgagor (the
         "Consent Letter"), there are no anticipated requests by Mortgagor
          --------------
         pursuant to the Mortgage for the consent or approval of Mortgagee, and
         (C) no events or circumstances have occurred that would have a
         material adverse effect on the Mortgaged Premises or the operation,
         management or profitability thereof.

                   (h)  There has been no material adverse change to Mortgagor
         from the date of the Consent Letter to the date of execution of this
         Agreement, and the principal terms of the Transfer and the proposed
         structure of Mortgagor as described in the Consent Letter have remained
         unchanged.

                   (i)  There have been no amendments, modifications or
         supplements to that certain Contribution Agreement, dated as of
         September 2, 1997 between Mortgagor and Original Mortgagor.

                   (j)  Mortgagor General Partner is the only Controlling Party
         of Mortgagor.

                   (k)  The reference in Section 5.01(e) of the Mortgage to "any
         warranty, representation or other statement made by or on behalf of
         Mortgagor in or pursuant to this Mortgage" shall include, without
         limitation, the representations and warranties of Mortgagor set forth
         in this Section 12 (as reconfirmed and restated) and in the
         Environmental Indemnity, the Corner Parcel

                                      -14-
<PAGE>
 
         Indemnity, any other 1997 Modification Document and in the Transfer
         Documents.

                   (l)  A representation or warranty made in this Section 12
         "to the best of Mortgagor's knowledge" means that Mortgagor shall be
         deemed to have made a reasonable investigation and inquiry with respect
         to such representation or warranty.

                   (m)  The meaning of the term "material", as used in this
         Section 12, shall be determined solely as of the date of this
         Agreement.

                   13.  Same Indebtedness; Maximum Principal Amount.
                        -------------------------------------------

                   (a)  Mortgagor and Mortgagee hereby certify that this
         Agreement secures the same indebtedness as is secured by the
         Consolidation Agreement as amended by the 1990 Modification Agreement,
         the 1992 Modification Agreement and the 1995 Modification Agreement,
         and no new or further indebtedness is secured by, or under any
         contingency may be secured by, the Mortgage.

                   (b)  Notwithstanding anything to the contrary contained
         herein or in the Mortgage, the maximum amount of principal
         indebtedness secured by the Mortgage at the time of execution hereof,
         or which under any contingency may become secured by the Mortgage at
         any time hereafter, is One Hundred Eighty Million and No/100 Dollars
         ($180,000,000).

                   (c)  This Mortgage, in addition to the maximum principal
         indebtedness described above, secures interest due under the Notes and
         Out-of-Pocket Costs expended by the Mortgagee. As used in this Section
         13, "Out-of-Pocket Costs" shall mean (i) any and all sums actually paid
         or required to be paid by the Mortgagee for real estate taxes, taxes on
         rents or rentals or insurance premiums as provided in the Mortgage, and
         any and all other sums actually paid or required to be paid by the
         Mortgagee pursuant to the terms of the Mortgage to protect and preserve
         the Mortgaged Premises or the Mortgagee's interest therein, (ii) any
         and all sums, including, without limitation, judgments, settlements or
         compromises, reasonable attorneys' fees and other costs and expenses,
         paid by the Mortgagee in connection with any suit, action, legal
         proceeding or dispute of any kind, in which the Mortgagee is a party or
         appears, arising from or related to the lien of the Mortgage, and (iii)
         any amount, cost or charges with respect to which the Mortgagee becomes
         subrogated, upon payment, in respect of any lien which may affect the
         validity or priority of the Mortgage, whether under recognized
         principles of law or equity, or under express statutory authority.

                                      -15-
<PAGE>
 
                   14.  No Waiver. Nothing contained in this Agreement shall be
                        ---------
         deemed to constitute a waiver by Mortgagee of its rights under the
         Mortgage if any payment is not made strictly in accordance with the
         terms of the Notes or the Mortgage, as modified hereby. Nothing
         contained in this Agreement shall be deemed to constitute a waiver by
         Mortgagee of any right or remedy available to Mortgagee arising out of
         a default by Mortgagor under the Notes or the Mortgage, as modified
         hereby.

                   15.  Ratification. Except as amended or modified hereby, the
                        ------------
         terms and conditions of the Mortgage and the Notes are hereby ratified
         and confirmed in their entirety.

                   16.  Successors and Assigns. This Agreement and all of the
                        ----------------------
         covenants herein shall be deemed to be covenants running with the land
         and shall bind and inure to the benefit of the parties hereto and their
         successors and permitted assigns.

                   17.  Counterparts. This Agreement may be executed in several
                        ------------
         counterparts, each of which shall constitute one and the same
         instrument.

                   18.  Lien Law. This Agreement is made subject to the trust
                        --------
         fund provisions of Section 13 of the New York Lien Law.

                   19.  Further Assurances. Mortgagor, at its expense, will
                        ------------------
         execute, acknowledge and deliver or cause to be executed, acknowledged
         and delivered all such instruments and take all such action as
         Mortgagee from time to time may reasonably request in order that
         Mortgagee may obtain the full benefits of this Agreement, the Notes,
         the Mortgage, the Consolidation Agreement, the Assignment of Rents, the
         Assignment of Leases, the Tax Processing Agreement, the Cash Collateral
         Agreement, the Environmental Indemnity, the Corner Parcel Indemnity and
         the other Loan Documents, and the full benefits of the rights, powers
         and remedies intended to be granted thereby.


                   20.  New York Law.
                        ------------

                   (a)  This Agreement shall be governed by and construed and
         enforced in accordance with the procedural and substantive laws of the
         State of New York. Any legal action or proceeding with respect to this
         Agreement must be brought in the courts of the State of New York within
         the First Judicial Department 1st District, New York City or of the
         United States of America for the Southern District of New York, and,
         each party hereto hereby accepts for itself and in respect of its
         property, generally and unconditionally, the jurisdiction of the
         aforesaid courts.

                                      -16-
<PAGE>
 
                   (b)  Each party hereto hereby irrevocably waives any
         objection which it may now or hereafter have to the laying of venue of
         any of the aforesaid actions or proceedings arising out of or in
         connection with this Agreement brought in the courts referred to in
         clause (a) above, and hereby further irrevocably waives and agrees not
         to plead or claim in any such court that any such action or proceeding
         brought in any such court has been brought in an inconvenient forum.
         Each party hereto further hereby waives the defense of sovereign
         immunity.


                   [Remainder of Page Intentionally Left Blank]

                                      -17-
<PAGE>
 
                   IN WITNESS WHEREOF, the parties hereto have caused this Note
         and Mortgage Modification and Spreader Agreement to be duly executed as
         of the day and year first above written.

                                      MORTGAGOR:

                                      BOSTON PROPERTIES LIMITED PARTNERSHIP,
                                        A Delaware Limited Partnership

                                      By:  Boston Properties, Inc.,
                                           General Partner

                                           By: /s/ Robert Selsam
                                              ------------------------------
                                                Name:  Robert Selsam
                                                Title: Senior Vice President


                                      MORTGAGEE:

                                      JOHN HANCOCK MUTUAL LIFE INSURANCE
                                        COMPANY, a Massachusetts
                                        corporation

                                      By: /s/ Debbie Orloff
                                         -----------------------------------
                                         Name:  Debbie Orloff
                                         Title: Investment Officer
<PAGE>
 
         STATE OF NEW YORK   )
                             :    ss:
         COUNTY OF NEW YORK  )



                   On this ___ day of November, 1997, before me personally came
         ____________________, to me known to be the person(s) who executed the
         foregoing instrument, and who, being duly sworn by me, did depose and
         say that he/she is the _________________ of Boston Properties, Inc., a
         Delaware corporation, the general partner of Boston Properties Limited
         Partnership, a limited partnership duly organized under the laws of
         the State of Delaware, having an address at 599 Lexington Avenue, New
         York, New York 10022; and that he/she has authority to sign the
         foregoing instrument for and on behalf of Boston Properties Limited
         Partnership, and he/she acknowledged to me that he/she executed the
         same as the act and deed of said partnership for the use and purposes
         therein mentioned.



                                        -------------------------------------
                                                 Notary Public
<PAGE>
 
         COMMONWEALTH OF MASSACHUSETTS )
                                       :    ss.:
         COUNTY OF SUFFOLK             )


                   On this ___ day of November, 1997, before me personally came
         _______, to me known to be the person who executed the foregoing
         instrument, and who, being duly sworn by me, did depose and say that he
         resides at ________________; that he is a ______________ of John
         Hancock Mutual Life Insurance Company, the corporation described in and
         which executed the foregoing instrument; that he knows the seal of said
         corporation; that the seal affixed to said instrument is such corporate
         seal; that it was so affixed by order of the Board of Directors of said
         corporation, and that he signed his name thereto by like order.



                                        -------------------------------------
                                                 Notary Public
