
<PAGE>   1

                                                                     EXHIBIT 5.1

                                December 3, 1998



AMB Property Corporation
AMB Property, L.P.
505 Montgomery Street
San Francisco, CA 94111

        Re:     AMB Property Corporation, a Maryland corporation (the
                "Company"); AMB Property, L.P., a Delaware limited partnership
                (the "Operating Partnership") - Registration Statement on Form
                S-3 pertaining to $400,000,000 maximum aggregate initial
                offering price of senior or subordinated debt securities of the
                Operating Partnership (the "Debt Securities") and guarantees of
                the Debt Securities by the Company ("Guarantees"), and
                $600,000,000 maximum aggregate initial offering amount of (i)
                shares of common stock of the Company, par value $.01 per share
                ("Common Stock"); (ii) shares of preferred stock of the Company,
                par value $.01 per share ("Preferred Stock"); (iii) depositary
                shares evidencing shares of Preferred Stock of the Company
                ("Depositary Shares"); and (iv) warrants to purchase shares of
                Common Stock or shares of Preferred Stock ("Warrants")


Ladies and Gentlemen:

       In connection with the registration of the Debt Securities, the
Guarantees, shares of Common Stock, shares of Preferred Stock, the Depositary
Shares and Warrants (collectively, the "Securities") under the Securities Act of
1933, as amended (the "Act"), by the Operating Partnership and the Company,
pursuant to a Registration Statement on Form S-3 to be filed with the Securities
and Exchange Commission (the "Commission") on or about December 2, 1998 (the
"Registration Statement"), you have requested our opinion with respect to the
matters set forth below. Capitalized terms not otherwise defined herein shall
have the meanings ascribed to them in the Registration Statement.



<PAGE>   2


BALLARD SPAHR ANDREWS & INGERSOLL, LLP

AMB Property Corporation
AMB Property, L.P.
December 3, 1998
Page 2

       We have acted as special Maryland corporate counsel to the Company in
connection with the matters described herein. With respect to such matters, the
Company acts in its individual capacity and in its capacity as general partner
of the Operating Partnership. In our capacity as special Maryland corporate
counsel to the Company, we have reviewed and are familiar with the charter of
the Company (the "Charter"), consisting of Articles of Incorporation filed with
the Maryland State Department of Assessments and Taxation (the "Department") on
November 24, 1997, and Articles Supplementary filed on July 23, 1998, November
12, 1998 and November 25, 1998; the Bylaws of the Company (the "Bylaws"), which
were duly adopted by the Board of Directors of the Company on November 24, 1997;
and certain resolutions adopted and actions taken by the Board of Directors of
the Company (the "Board of Directors") on or before the date hereof and in full
force and effect on the date hereof including, but not limited to, those certain
resolutions adopted by the Board of Directors on November 18, 1998. We have also
examined the Registration Statement, other documents, corporate and other
records of the Company and certificates of public officials and officers of the
Company including, without limitation, a status certificate of recent date
issued by the Department to the effect that the Company is duly incorporated and
existing under the laws of the State of Maryland, and a Certificate of Officers
of the Company of recent date to the effect that, among other things, the
Charter and Bylaws of the Company and the resolutions and actions by the Board
of Directors which we have examined are true, correct and complete, have not
been rescinded or modified and are in full force and effect on the date of such
certificate. We have also made such further legal and factual examinations as we
have deemed necessary or appropriate to provide a basis for the opinion set
forth below.

       In reaching the opinions set forth below, we have assumed the following:
(a) each person executing any instrument, document or agreement on behalf of any
party (other than the Company or the Operating Partnership) is duly authorized
to do so; (b) each natural person executing any instrument, document or
agreement is legally competent to do so; (c) all documents submitted to us as
originals are authentic; all documents submitted to us as certified, facsimile
or photostatic copies conform to the original document; all signatures on all
documents submitted to us for examination are genuine; and all public records
reviewed are accurate and complete; (d) the resolutions adopted and to be
adopted, and the actions taken and to be taken by the Board of Directors
including, but not limited to, the adoption of all resolutions and the taking of
all action necessary to authorize the issuance and sale of the Securities and
the making of the Guarantees in accordance with the procedures set forth in
paragraphs 1, 2, 3 and 4 below, have occurred or will occur at duly called
meetings at which a quorum of the incumbent directors of the Company were or are
present and acting throughout, or by unanimous written consent of all incumbent
directors, all in accordance with the Charter and Bylaws of the Company and



<PAGE>   3

BALLARD SPAHR ANDREWS & INGERSOLL, LLP

AMB Property Corporation
AMB Property, L.P.
December 3, 1998
Page 3


applicable law; (e) the number of shares of Preferred Stock (including without
limitation any shares of Preferred Stock evidenced by Depositary Shares) and the
number of shares of Common Stock to be offered and sold under the Registration
Statement, together with the number of shares of Preferred Stock and the number
of shares of Common Stock issuable upon the conversion or exchange of Preferred
Stock or the exercise of the Warrants, will not, in the aggregate, exceed the
number of shares of Preferred Stock, and the number of shares of Common Stock,
respectively, authorized in the Charter of the Company, less the number of
shares of Preferred Stock and the number of shares of Common Stock,
respectively, authorized and reserved for issuance and/or issued and outstanding
on the date on which the Securities are authorized, the date on which the
Securities are issued and delivered, the date on which the Warrants are
exercised and the date on which shares of Preferred Stock and shares of Common
Stock, respectively, are issued in connection with the issuance of Depositary
Shares or pursuant to the conversion or exchange of Preferred Stock or the
exercise of Warrants; (f) none of the terms of any of the Securities, or any
agreements related thereto, to be established subsequent to the date hereof, nor
the issuance and delivery of any such Securities nor the compliance by the
Company with the terms of any such Securities or agreements will violate any
applicable law or will conflict with, or result in a breach or violation of, the
Charter or Bylaws of the Company, or any instrument or agreement to which the
Company is a party or by which the Company is bound or any order or decree of
any court, administrative or governmental body having jurisdiction over the
Company; (g) the form of certificate or other instrument or document
representing the Securities will conform in all respects to the requirements
applicable under Maryland law; and (h) none of the Securities, and none of the
shares of Preferred Stock or shares of Common Stock issuable upon the conversion
or exchange of Preferred Stock or exercise of the Warrants, or the Depositary
Shares or related Depositary Receipts, will be issued and sold in violation of
the provisions of Article IV, Section E of the Charter of the Company entitled
"Restrictions on Ownership and Transfer to Preserve Tax Benefits."

       Based on the foregoing, and subject to the assumptions and qualifications
set forth herein, it is our opinion that:

        1. Upon due authorization by the Board of Directors of a designated
number of shares of Common Stock for issuance at a minimum price or value of
consideration to be set by the Board of Directors, all necessary corporate
action on the part of the Company will have been taken to authorize the issuance
and sale of such shares of Common Stock, and when such shares of Common Stock
are issued and delivered against payment of the consideration therefor as set by
the Board of Directors, whether upon original issue or upon exchange or
conversion of duly authorized and validly issued shares of Preferred Stock or
the exercise of duly authorized and 



<PAGE>   4


BALLARD SPAHR ANDREWS & INGERSOLL, LLP

AMB Property Corporation
AMB Property, L.P.
December 3, 1998
Page 4

valid Warrants, such shares of Common Stock will be validly issued, fully paid
and non-assessable.

        2. Upon (a) designation by the Board of Directors of one or more classes
of Preferred Stock to distinguish each such class from other then outstanding
classes of Preferred Stock; (b) setting by the Board of Directors of the number
of shares of Preferred Stock to be included in each such class; (c)
establishment by the Board of Directors of the preferences, conversion and other
rights, voting powers, restrictions, limitations as to dividends, qualifications
and terms and conditions of redemption of each such class of Preferred Stock;
(d) filing by the Company with the Department of Articles Supplementary setting
forth a description of each such class of Preferred Stock, including the
preferences, conversion and other rights, voting powers, restrictions,
limitations as to dividends, qualifications and terms and conditions of
redemption as set by the Board of Directors and a statement that the Preferred
Stock has been classified by the Board of Directors under the authority
contained in the Charter, and the acceptance for record by the Department of
such Articles Supplementary; and (e) due authorization by the Board of Directors
of a designated number of shares of Preferred Stock for issuance at a minimum
price or value of consideration to be set by the Board of Directors: all
necessary corporate action on the part of the Company will have been taken to
authorize the issuance and sale of such shares of Preferred Stock and when such
shares of Preferred Stock are issued and delivered against payment of the
consideration therefor as set by the Board of Directors, whether upon original
issue or upon exchange or conversion of other shares of duly authorized and
validly issued Preferred Stock or the exercise of duly authorized and valid
Warrants, such shares of Preferred Stock will be validly issued, fully paid and
non-assessable.

        3. Upon (a) designation and titling by the Board of Directors of the
Depositary Shares; (b) setting by the Board of Directors of the number of
Depositary Shares to be issued; (c) establishment by the Board of Directors of
the terms, conditions and provisions of the Depositary Shares and any related
agreements; (d) due authorization by the Board of Directors of the execution and
delivery of the Deposit Agreement relating to the Depositary Shares and the
issuance of the Depositary Shares at a minimum price or value of consideration;
and (e) due authorization and issuance by the Corporation of the shares of
Preferred Stock of the Company to be evidenced by the Depositary Shares in
accordance with the procedures set forth in Paragraph 2 above: all necessary
corporate action on the part of the Company will have been taken to authorize
the issuance and sale of the Depositary Shares, and when certificates
representing the shares of Preferred Stock to be represented by the Depositary
Shares are duly executed and delivered to the Depositary by the Company against
payment therefor in accordance with the Deposit Agreement in the manner
contemplated by the Registration Statement and/or the 



<PAGE>   5




BALLARD SPAHR ANDREWS & INGERSOLL, LLP

AMB Property Corporation
AMB Property, L.P.
December 3, 1998
Page 5

applicable Prospectus Supplement, such shares of Preferred Stock represented by
the Depositary Shares will be validly issued, fully paid and nonassessable.

        4. Upon (a) due authorization by the Board of Directors of the issuance,
execution and delivery by the Corporation in its capacity as general partner of
the Operating Partnership of the Debt Securities and the execution and delivery
by the Corporation in its individual capacity and in its capacity as general
partner of the Operating Partnership and on behalf of the Operating Partnership,
as the case may be, of the Guarantees and/or any necessary and appropriate
supplements, amendments or modifications to the Indenture (inclusive therein of
Guarantees providing for the guaranty by the Company of the obligations of the
Operating Partnership under the Debt Securities) (collectively, the
"Supplements"); and (b) the establishment of the terms, conditions and
provisions of the Debt Securities, the Guarantees and the Supplements by the
Board of Directors or a duly authorized officer of the Corporation, acting on
behalf of the Corporation in its individual capacity or in its capacity as
general partner of the Operating Partnership, as the case may be: the execution,
delivery and performance of the Indenture as supplemented, amended or modified
by the Supplements (inclusive of the Guarantees provided for therein) and the
Guarantees will have been duly authorized by all necessary corporate action on
the part of the Corporation acting in its individual capacity and in its
capacity as general partner of the Operating Partnership, as the case may be,
and the issuance of the Debt Securities will have been duly authorized by all
necessary corporate action on the part of the Corporation acting in its capacity
as general partner of the Operating Partnership.

       This opinion is limited to the present corporate laws of the State of
Maryland and we express no opinion with respect to the laws of any other
jurisdiction. Furthermore, the opinions presented in this letter are limited to
the matters specifically set forth herein and no other opinion shall be inferred
beyond the matters expressly set forth herein. Without limiting the generality
of the foregoing, we express no opinion with respect to any securities laws or
with respect to the action required for the Operating Partnership to authorize,
execute or deliver any of the Securities or any other document, instrument or
agreement.

       The opinions set forth in this letter are rendered as of the date hereof
and are necessarily limited to laws now in effect and facts and circumstances
presently existing and brought to our attention. We assume no obligation to
supplement this opinion if any applicable law is changed after the date hereof
or if we become aware of any facts or circumstances which now exist or which
occur or arise in the future and may change the opinions expressed herein after
the date hereof.



<PAGE>   6

BALLARD SPAHR ANDREWS & INGERSOLL, LLP

AMB Property Corporation
AMB Property, L.P.
December 3, 1998
Page 6


       We consent to the filing of this opinion as an exhibit to the
Registration Statement and further consent to the filing of this opinion as an
exhibit to applications to the securities commissioners of the various states of
the United States for registration of the Securities. We also consent to the
identification of our firm as Maryland counsel to the Company in the section of
the Prospectus (which is a part of the Registration Statement) entitled "Legal
Matters." In giving these consents, we do not admit that we are within the
category of persons whose consent is required by Section 7 of the Act.

       The opinions expressed in this letter are for your use and the use of
your securities counsel, Latham & Watkins, in connection with the filing of the
Registration Statement and the rendering of opinions by Latham & Watkins in
connection therewith, and may not be relied upon by you or Latham & Watkins for
any other purpose, without our prior written consent.

                                      Very truly yours,

                                      /s/ Ballard Spahr Andrews & Ingersoll, LLP




