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            [FRIED, FRANK, HARRIS, SHRIVER & JACOBSON LETTERHEAD]

                                                                     EXHIBIT 5.1


                               January 4, 2000

Board of Directors
CoStar Group, Inc.
7475 Wisconsin Avenue
Bethesda, Maryland 20814

Ladies and Gentlemen:

       We have acted as special counsel for CoStar Group, Inc., a Delaware
corporation (the "Company") and its wholly owned subsidiary, Acq Sub, Inc., a
Delaware corporation ("Merger Sub"), in connection with the merger into Merger
Sub of Comps.com pursuant to the Agreement and Plan of Merger, dated as of
November 3, 1999 (the "Merger Agreement"), by and among the Parent, Merger Sub
and the Comps.com. Under the terms of the Merger Agreement, the Company expects
to issue up to 2,242,000 shares of its common stock (the "Merger Shares") to
holders of common stock of Comps.com.

       With your permission, all assumptions and statements of reliance herein
have been made without any independent investigation or verification on our part
except to the extent otherwise expressly stated, and we express no opinion with
respect to the subject matter or accuracy of such assumptions or items relied
upon.

       In connection with this opinion, we have (i) investigated such questions
of law, (ii) examined originals or certified, conformed or reproduction copies
of such agreements, instruments, documents and records of the Company, such
certificates of public officials and such other documents, and (iii) received
such information from officers and representatives of the Company as we have
deemed necessary or appropriate for the purposes of this opinion. In all
examinations, we have assumed the legal capacity of all natural persons
executing documents, the genuineness of all signatures, the authenticity of
original and certified documents and the conformity to original or certified
copies of all copies submitted to us as conformed or reproduction copies. As to
various questions of fact relevant to the opinions expressed herein, we have
relied upon, and assume the accuracy of, representations and warranties
contained in the documents and certificates and oral or written statements and
other information of or from representatives of the Company and others and
assume compliance on the part of all parties to the documents with their
covenants and agreements contained therein.

       Based upon the foregoing and subject to the limitations, qualifications
and assumptions set forth herein, we are of the opinion that the Merger Shares
registered under the Registration Statement, when issued, delivered, and paid
for in accordance with


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the terms of the Merger Agreement, will be duly authorized, legally issued,
fully paid and non-assessable.

       The opinion expressed herein is limited to the General Corporation Law of
the State of Delaware, the Delaware Constitution and reported judicial decisions
interpreting those laws, each as currently in effect.

       We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the Prospectus forming part of the Registration Statement. In
giving such consent, we do not hereby admit that we are in the category of such
persons whose consent is required under Section 7 of the Securities Act of 1933.


                                               Very truly yours,

                               FRIED, FRANK, HARRIS, SHRIVER & JACOBSON


                               By:   /s/
                                  ----------------------------------------------
                                                    Stephen I. Glover