EXHIBIT 23.1

Consent of Independent Registered Public Accounting Firm

     We consent to the incorporation by reference in the Registration Statement (Form S-8 No. 333-45770) pertaining to the 1998 Stock Incentive Plan of CoStar Group, Inc. of our reports dated February 19, 2007 with respect to the consolidated financial statements of CoStar Group, Inc., CoStar Group, Inc. management’s assessment of the effectiveness of internal control over financial reporting, 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, 2006.

Ernst & Young LLP

McLean, Virginia
February 26, 2007