

<PAGE>

                                                                    Exhibit 23.1



                       CONSENT OF INDEPENDENT ACCOUNTANTS

As independent public accountants, we hereby consent to the incorporation of our
report dated March 24, 1998 in this Form 8-K on the combined statement of
revenue and certain expenses of Three Christina Centre and our report dated
April 15, 1998 in this Form 8-K on the combined statement of revenue and certain
expenses of the DKM Properties into the Company's previously filed Registration
Statements on Forms S-3 (File No. 333-46647, File No. 333-39155 and File No.
333-20999) and Forms S-8 (File No. 333-14243 and File No. 333-28427).

                                                             ARTHUR ANDERSEN LLP

Philadelphia, Pa.,
     April 16, 1998



