<Page>
Exhibit 5.1
GOODWIN PROCTER LLP
COUNSELLORS AT LAW
EXCHANGE PLACE
BOSTON, MASSACHUSETTS 02109
March 6, 2003
Boston Properties Limited Partnership
111 Huntington Avenue
Boston, Massachusetts 02199
Ladies and Gentlemen:
We have acted as counsel for Boston Properties Limited Partnership, a Delaware
limited partnership (the "Company"), and Boston Properties, Inc., a Delaware
corporation and general partner of the Company (the "General Partner"), in
connection with the preparation and filing with the Securities and Exchange
Commission (the "SEC") under the Securities Act of 1933, as amended (the
"Securities Act"), of the Registration Statement on Form S-4 (the "Registration
Statement"), relating to the registration of the offer by the Company to
exchange up to $925 million aggregate principal amount of its 6.25% Senior Notes
due 2013 (the "New Notes") for its existing 6.25% Senior Notes due 2013 (the
"Old Notes"). The New Notes are proposed to be issued in accordance with the
provisions of the Indenture dated as of December 13, 2002, between the Company
and The Bank of New York, as Trustee (the "Trustee"), as supplemented by a first
supplemental indenture dated December 13, 2002 and a second supplemental
indenture dated January 17, 2003 (collectively, the "Indenture") as contemplated
by (i) the Registration Rights Agreement, dated as of December 10, 2002, by and
among the Company and J.P. Morgan Securities Inc., Banc of America Securities
LLC, Salomon Smith Barney Inc., BNY Capital Markets, Inc., Commerzbank Capital
Markets Corp., Deutsch Bank Securities Inc., Goldman, Sachs & Co., McDonald
Investments Inc. and Morgan Stanley & Co. Incorporated and (ii) the Registration
Rights Agreement, dated as of January 13, 2003, by and among the Company and
Banc of America Securities LLC, J.P. Morgan Securities Inc. and Salomon Smith
Barney Inc. (collectively, the "Registration Rights Agreements").
In connection with this opinion, we have examined originals or copies, certified
or otherwise identified to our satisfaction, of (i) the Registration Statement,
including the form of prospectus included therein and the documents incorporated
by reference therein, (ii) the Company's Second Amended and Restated Certificate
of Limited Partnership, as amended to date, (iii) such records of the corporate
proceedings of the Company as we deemed material, (iv) the Indenture, (v) the
Registration Rights Agreements, (vi) the Form T-1 of the Trustee filed as an
exhibit to the Registration Statement and (vii) the form of the New Notes. In
addition, we have examined and relied on the originals or copies, certified or
otherwise identified to our satisfaction, of such other corporate and
partnership records of the Company, and such other instruments and other
certificates of public officials, officers and representatives of the Company
and other persons, and we have made such investigations of law, as we have
deemed appropriate as a basis for the opinions expressed below.
Boston Properties Limited Partnership
March 6, 2003
Page 2
In rendering the opinions expressed below, we have assumed and have not verified
the genuineness of all signatures, the authenticity of all documents submitted
to us as originals, the conformity to original documents of all documents
submitted to us as certified or photostatic copies, the authenticity of the
originals of such latter documents, the legal capacity of each individual
executing any document, and the factual accuracy and completeness of all
representations, warranties and other statements made by the parties. We have
also assumed that prior to the delivery of any New Notes, the Registration
Statement will have been declared effective.
We are members of the Bar of the Commonwealth of Massachusetts, and in rendering
the opinions expressed below, we express no opinion other than as to the laws of
the United States and the Commonwealth of Massachusetts, the Delaware General
Corporation Law, the Delaware Limited Liability Company Act and the Delaware
Revised Uniform Limited Partnership Act (collectively, the "Delaware Statutes")
and the applicable provisions of the Delaware Constitution and reported judicial
decisions interpreting the Delaware Statutes. To the extent that any other laws
govern any of the matters as to which we express an opinion herein, we have
assumed, without independent investigation, that the laws of such jurisdiction
are identical to those of the Commonwealth of Massachusetts, and we express no
opinion as to whether such assumption is reasonable or correct.
Based on and subject to the foregoing, we are of the opinion that when the New
Notes (in the form examined by us) have been duly executed by the Company,
authenticated by the Trustee in accordance with the terms of the Indenture and
issued and delivered upon consummation of the Exchange Offer against receipt of
Old Notes surrendered in exchange therefor in accordance with the terms of the
Exchange Offer, the Registration Rights Agreements, the Registration Statement
and the Indenture, the New Notes will constitute valid and binding obligations
of the Company, enforceable against the Company in accordance with their terms,
except as may be limited by (i) bankruptcy, insolvency, receivership,
reorganization, liquidation, voidable preference, moratorium, fraudulent
transfer or conveyance or other similar laws now or hereafter in effect
affecting creditors' rights generally and (ii) general principles of equity
including without limitation reasonableness, materiality, good faith and fair
dealing (regardless of whether considered in a proceeding at law or in equity)
and to the effect of certain laws and judicial decisions upon the availability
and enforceability of certain remedies, including without limitation the remedy
of specific performance.
This opinion is based upon currently existing statutes, rules and regulations
and judicial decisions and is rendered as of the date hereof, and we disclaim
any obligation to advise you of any change in any of the foregoing sources of
law or subsequent developments in law or changes in facts or circumstances which
might affect any matters or opinions set forth herein.
We consent to the filing of this opinion as an exhibit to the Registration
Statement, and to the reference to our firm under the heading "Legal Matters" in
the prospectus which is part of the Registration Statement.
Boston Properties Limited Partnership
March 6, 2003
Page 3
Very truly yours,
/s/ GOODWIN PROCTER LLP
- ---------------------------------------
GOODWIN PROCTER LLP