EXHIBIT
23.1
Consent
of Independent Registered Public Accounting Firm
We
consent to the incorporation by reference in Registration Statements on Form S-8
Nos. 333-82599, 333-92165, 333-45770, 333-69548, 333-135709 and 333-143968 of
our reports dated February 22, 2008 with respect to the consolidated financial
statements of CoStar Group, Inc. and the effectiveness of internal control over
financial reporting of CoStar Group, Inc., included in this Annual Report (Form
10-K) for the year ended December 31, 2007, filed with the Securities and
Exchange Commission.
/s/
Ernst & Young LLP
McLean,
Virginia
February
22, 2008