<PAGE>   1
                                                                     EXHIBIT 5.1
                         BROBECK, PHLEGER & HARRISON LLP

                                November 26, 1996



Sunstone Hotel Investors, Inc.
115 Calle de Industrias
Suite 201
San Clemente, California 02672

         Re:      Sunstone Hotel Investors, Inc. (the "Company") - Registration
                  Statement on Form S-3 pertaining to $200,000,000 maximum
                  aggregate initial offering price 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"); and (iii) warrants to purchase
                  shares of Common Stock or shares of Preferred Stock
                  ("Warrants")

Ladies and Gentlemen:

                  We have acted as counsel to the Company in connection with the
Company's registration of shares of Common Stock, shares of Preferred Stock, and
Warrants (collectively the "Securities") under the Securities Act of 1933, as
amended (the "Act") by the Company on Form S-3, transmitted for filing with the
Securities and Exchange Commission (the "Commission") on the date hereof (the
"Registration Statement"). Capitalized terms not otherwise defined herein shall
have the meanings ascribed to them in the Registration Statement.

                  In rendering this opinion, we have examined and relied upon a
copy of the Registration Statement. We have also examined originals, or copies
of originals certified to our satisfaction, of such agreements, documents,
certificates and other statements of governmental officials and other
instruments, and have examined such questions of law and have satisfied
ourselves as to such matters of fact, as we have considered relevant and
necessary as a basis for this opinion. We have assumed the authenticity of all
documents submitted to us as originals, the genuineness of all signatures, the
legal capacity of all natural persons and the conformity with the original
documents of any copies thereof submitted to us for our examination.

                  Based on the foregoing, it is our opinion that:

                  1. The shares of Common Stock will be legally issued, fully
paid and non-assessable (assuming that the requisite number of authorized but
unissued shares of Common Stock then exist) when (i) the Registration Statement,
as finally amended (including any necessary post-effective amendments), shall
have become effective under the Securities Act; (ii) a Prospectus Supplement
with respect to such shares of Common Stock shall have been filed (or
<PAGE>   2
transmitted for filing) with the Commission pursuant to Rule 424 under the
Securities Act; (iii) the Company's Board of Directors or a duly authorized
committee thereof shall have adopted final resolutions authorizing the issuance
and sale of such shares of Common Stock in an amount not exceeding the
authorized but unissued capital stock of the Company, and as contemplated by the
Registration Statement and a Prospectus Supplement relating thereto; and (iv)
certificates representing the shares of Common Stock shall have been duly
executed, countersigned and registered and duly delivered to the purchasers
thereof (or if such shares are uncertificated, all appropriate action has been
taken to register such shares in book-entry form) against payment of the agreed
consideration therefore (which consideration will not be less than the $.01 par
value per share), as provided in the Registration Statement and any Prospectus
Supplement relating thereto and the final authorizing resolution.

                  2. The shares of Preferred Stock will be legally issued, fully
paid and non-assessable (assuming that the requisite number of authorized but
unissued shares of Preferred Stock then exists) when (i) the Registration
Statement, as finally amended (including any necessary post-effective
amendments), shall have become effective under the Securities Act; (ii) a
Prospectus Supplement with respect to such shares of Preferred Stock shall have
been filed (or transmitted for filing) with the Commission pursuant to Rule 424
under the Securities Act; (iii) the Company's Board of Directors or a duly
authorized committee thereof, shall have duly adopted final resolutions setting
forth the terms of any particular series of Preferred Stock and authorizing the
issuance and sale of such shares of Preferred Stock in an amount not exceeding
the authorized but unissued capital stock of the Company, and as contemplated by
the Registration Statement and any Prospectus Supplement relating thereto; (iv)
the Articles Supplementary setting forth the terms of such series of Preferred
Stock, including setting a quantity of unissued shares Preferred Stock as will
permit the issuance of the shares of Preferred Stock authorized for issuance and
setting forth a description of such shares of Preferred Stock, including the
references, conversion and other rights, voting powers, restrictions,
limitations as to dividends, qualifications and terms and conditions of
redemption, consistent with the final authorizing resolution shall have been
filed with, and accepted for record by, the Department of Assessments and
Taxation of the State of Maryland; and (v) certificates representing the shares
of Preferred Stock shall have been duly executed, countersigned and registered
and duly delivered to the purchasers thereof (or if such shares are
uncertificated, all appropriate action has been taken to register such shares in
book-entry form) against payment of the agreed consideration therefor (which
consideration will not be less than the $.01 par value per share), as provided
in the Registration Statement and any Prospectus Supplement relating thereto and
the final authorizing resolution.

                  3. The Warrants will be duly authorized and legally issued and
valid and binding obligations of the Company (except to the extent
enforceability may be limited by applicable bankruptcy, insolvency,
reorganization, moratorium, fraudulent transfer or other similar laws affecting
the enforcement of creditors' rights generally and by the effect of general
principles of equity, regardless of whether enforceability is considered in a
proceeding in equity or at law) when (i) the Registration Statement, as finally
amended (including any necessary post-effective amendments), shall have become
effective under the Securities Act; (iii) a Prospectus Supplement with respect
to such Warrants shall have been filed (or transmitted for filing) the
Commission pursuant to Rule 424 under the Securities Act; (iii) the Company's
Board of Directors or a duly authorized committee thereof shall have duly
adopted final resolutions
<PAGE>   3
Sunstone Hotel Investors, Inc. 
November 26, 1996
Page 4


setting forth the terms of any particular series of Warrants, authorizing the
issuance and sale of the Warrants in an amount that upon exercise will not
exceed the authorized capital stock of the Company, if applicable, and as
contemplated by the Registration Statement and any Prospectus Supplement
relating thereto and approving one or more warrant agreements (each, a "Warrant
Agreement"), to be among the Company, and a financial institution identified
therein as warrant agent (each, a "Warrant Agent"), establishing the terms of
such Warrants; and (iv) the Warrants shall have been duly executed,
authenticated by the Warrant Agent and registered and duly delivered to the
purchasers thereof against payment of the agreed consideration therefor, as
provided in the Registration Statement and any Prospectus Supplement relating
thereto and the final authorizing resolution and the Warrant Agreement
applicable thereto.

                  We do not find it necessary for the purposes of this opinion
to cover, and accordingly we express no opinion as to, the application of the
securities or blue sky laws of the various states to the sale of the Securities.

                  Except as expressly stated in the next sentence, this opinion
is limited to the laws of the State of California and the laws of the United
States of America, to the extent applicable. Insofar as the opinions expressed
above relate to matters governed by the laws of the State of Maryland, we have
not made an independent examination of such laws, but have relied exclusively as
to such laws, subject to the exceptions, qualifications and limitations therein
expressed, upon the opinion of Ballard, Spahr, Andrews & Ingersoll LLP of
Baltimore, Maryland filed as Exhibit 5.2 to the Registration Statement.

                  We hereby consent to the filing of this opinion as an Exhibit
to the Registration Statement and to all references to our firm included in or
made a part of the Registration Statement.

                                              Very truly yours,

                                        /s/   BROBECK, PHLEGER & HARRISON LLP