Additional Proxy Soliciting Materials (Form DEFA14A)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No.  )

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

COSTAR GROUP, INC.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

CoStar Group, Inc.

Supplemental Information Regarding Proposal 3: Approval of, on a Non-Binding Advisory Basis, the Compensation of the Company's Named Executive Officers

On May 18, 2026, CoStar Group, Inc. (the "Company") made available to stockholders a presentation that the Company intends to use from time to time during meetings with stockholders, in connection with its upcoming annual meeting of stockholders to be held on June 23, 2026. A copy of the presentation is provided on the pages that follow.

This supplemental information should be read together with the Company's definitive proxy statement filed with the Securities and Exchange Commission on April 30, 2026.

2026 Meeting Annual Stockholder Engagement May 18, 2026

Forward-Looking Statements This presentation includes "forward-looking statements." Potential factors that may cause actual results to differ materially from those projected in these forward-looking statements are described in CoStar Group's periodic filings with the Securities and Exchange Commission ("SEC"), such as Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, and Current Reports on Form 8-K, including the "Risk Factors" sections of those documents. These filings can be accessed via the SEC's website at www.sec.gov. All forward-looking statements reflect information available to CoStar Group as of the date of this presentation, and CoStar Group undertakes no obligation to update or revise any forward-looking statements unless required by applicable law. 

2026 Board Nominees RECENT ADDITIONS DIRECTOR OTHER PUBLIC NAME AND PRINCIPAL OCCUPATION AGE A C NCG 2025 SINCE COMPANY BOARDS Andrew C. Florance Founder and Chief Executive Officer, 62 1987 0 Berisford CoStar Group, Inc. Mr. Berisford brings his extensive Louise S. Sams leadership experience in data Board Chair, CoStar Group, Inc.; Former EVP & analytics, global information services, General Counsel, Turner Broadcasting System, Inc. 68 2019 2 and human capital strategy. Independent John L. Berisford Glaser Independent Former President, S&P Global Ratings 62 2025 0 Ms. Glaser brings nearly four decades of financial leadership, corporate development, and marketplace Angelique G. Brunner experience. Chief Executive Officer and Founder, EB5 Capital 54 2023 0 Independent Rachel C. Glaser McCarthy Former Chief Financial Officer, Etsy, Inc. 64 2025 1 Independent Ms. McCarthy brings more than 30 years of financial leadership, capital John W. Hill deployment, and enterprise Founder and Chief Executive Officer, J Hill Group technology governance experience. Independent 71 2012 0 2023 Christine M. McCarthy Former Executive Vice President and Chief Financial 70 2025 2 Officer, The Walt Disney Company Brunner Independent Ms. Brunner brings over 25 years' Robert W. Musslewhite experience in real estate investment, Former Chief Executive Officer, Definitive Healthcare Corp 56 2019 1 capital markets, and corporate Independent governance. COMMITTEES A Audit C Compensation NCG Nominating and Corporate Governance Chair Member

Director Nominees Skills & Experience Our director nominees have a broad range of skills and experiences, including some or significant experiences in the following: Industry/Commercial Real Estate Talent Management and Executive Compensation Public Company Governance Marketing and Sales Global Business Risk Management Financial Accounting and Reporting Senior Management and Leadership Legal, Public Policy, and Regulatory Cybersecurity and Data Privacy Business Development and M&A Corporate Responsibility and Sustainability AI/Generative AI Strategy and Governance Very Skilled/Experienced Some Skill/Experience No Experience 

Key Director Attributes AGE GENDER BOARD REFRESHMENT 2 2 63 50% 50% 4 4 Average Refreshment Women Years Since 2023 4 50s 60s 70s Women Men New Directors since 2023 Director from before 2023 TENURE INDEPENDENCE LEADERSHIP 1 2 4.86 88% 100% Average 4 Independent C-Suite Independent Experience Years 2 0-3 Years 3-7 Years >7 Years Independent Non-Independent C-Suite Experience No C-Suite Experience *Key Director Attributes are as of April 28, 2026.

Board Oversight Over Capital Allocation • In 2025, the Board established the Capital Allocation Committee to review and make recommendations regarding capital structure, capital allocation priorities, and financial targets. The Capital Allocation Committee reviewed: ✓ Strategy review for business ✓ Long-range financial plan for CoStar Group ✓ Operational and financial performance data for Homes.com ✓ Detailed long-range financial plan for Homes.com including breakeven and NPV analysis ✓ Cash flow analysis to determine degrees of freedom with respect to incremental capital return ✓ Historical IRR analysis of businesses • Committee members were Andy Florance (Chair), John Berisford, Christine McCarthy, and Robert Musslewhite • John Berisford and Christine McCarthy were nominated by activist stockholders • Committee unanimously recommended to the full board the 2026 budget, share repurchase program, and long-range Homes.com plan 6 Committee Meetings 5/9/2025 Inaugural Capital Allocation Committee Meeting Discussed committee mandate, authority, responsibilities, and expectations 6/5/2025 Committee Meeting Discussed potential areas of focus following Board orientation sessions 9/8/2025 Committee Meeting Discussed capital allocation framework and information required 9/22/2025 Committee Meeting Discussed the outline of Investor Presentation 11/11/2025 Committee Meeting Discussed Homes.com plan, Investor Presentation, and share repurchase program 12/16/2025 Committee Meeting Reviewed and finalized recommendations to the Board • The full Board unanimously approved the Committee's recommendations and oversees capital allocation matters since the completion of the Committee's mandate in January 2026 

Stockholder Engagement We regularly engage with our stockholders to understand their perspectives on our business and executive compensation program. During 2025, our management team met with 92% of our top 25 stockholders to discuss Company performance, our products and services, capital allocation matters, corporate governance, and executive compensation. 14 Investor Roadshows 92% 84% CoStar C-Suite 610 of outstanding in meetings shares of active 26 with Top 25 Institutional Investor Firms stockholders Stockholders Conferences

Stockholder Responsiveness Following extensive engagement with our stockholders during 2025, the Company implemented several enhancements to its governance and capital allocation profile and took actions to return capital to its stockholders, as outlined below. Strong Governance And Regular Board Disciplined Investments Across CoStar Deepening Commitment To Returning Refreshment Group Capital To Stockholders Added three independent directors, two Formed a Capital Allocation Committee Accelerated the completion of a $500 of whom were designated by D.E. Shaw comprised of independent directors million share repurchase program and Third Point. (including D.E. Shaw's and Third Point's initiated in 2025. designees) to support a comprehensive review of our capital structure, capital ITEMS allocation priorities, and financial targets. Appointed a new independent Board Developed a plan unanimously approved by Authorized a new $1.5 billion Chair. the Capital Allocation Committee to repurchase program in January 2026. ACTION moderate investment in Homes.com by $300 million in 2026 and $100+ million annually thereafter to achieve break-even exiting 2029. Acted on stockholder feedback, approved Deployed AI technology initiatives across the a redesigned executive compensation entire CoStar Group organization and program for 2026, featuring more enhancing and expanding commercial rigorous and quantitative goals, enhanced product offerings. transparency, and a simplified structure. 

Say-on-Pay Stockholder Engagement In addition, our Board Chair and Compensation Committee Chair, supported by our management team, met with stockholders in response to the 2025 Say-On-Pay vote. STOCKHOLDERS CONTACTED STOCKHOLDERS PROVIDING FEEDBACK STOCKHOLDERS CONTACTED REPRESENTING 77% of Top 50 of Top 50 of Total Outstanding Shares 100% Stockholders 42% Stockholders Contacted Responded FEEDBACK PROVIDED 33% of Total Outstanding Shares BOARD CHAIR OR COMPENSATION COMMITTEE CHAIR PARTICIPATION Provided Feedback BOARD AND/OR COMPENSATION COMMITTEE CHAIR PARTICIPATION of Top 50 who 100% Stockholders 100% requested 76% Offered of Stockholders that responded to outreach 

Board Acted on Stockholder Feedback on Executive Compensation WHAT WE HEARD CHANGED WHAT WE DID FOR 2026? Established symmetrical slopes for performance metrics, increased relative Enhance Goal Rigor in setting executive compensation targets TSR (rTSR) target and capped rTSR payout based on absolute TSR Reduce Weighting of Qualitative Goals of short-term incentives Increased the portion of short-term incentive payout based on objective for non-CEO NEOs financial metrics from 50-65% for 2025 to 80% for 2026 Give Heavier Weighting to PSUs as preferred vehicle for Set LTI mix of 80% PSUs for the CEO and 60% PSUs for the other executive stockholders officers Update Performance Metrics to ensure alignment with value Balanced EBITDA with Organic Revenue metric for STI metrics and creation, including TSR and per-share metrics replaced Revenue with EPS and rTSR for LTI metrics Eliminated stock options and annual Performance-Based Restricted Stock Simplify the Design of executive compensation structures in favor of a mix of PSUs and time-based RSUs Enhance Disclosures on compensation framework and the Committed to updating disclosures to provide greater clarity and detail Compensation Committee's decision making Remove Tax Gross-up from CEO's employment agreement Eliminated legacy 280G excise tax gross-up Supplemented existing clawback policy to, among other things, include Enhance Clawback Policy time-based RSUs Increase CEO's Stock Ownership Guidelines Increased minimum stock ownership requirements for CEO from 6x to 10x base salary Quantum of CEO Pay review against stockholder expectations We expect the quantum of CEO pay to more closely align with stockholder expectations following the changes described above

Executive Compensation Redesign THE COMPENSATION COMMITTEE SOUGHT TO: THE REVISED DESIGN: • Reinforce a strong pay-for-performance • Emphasizes rigorous and ambitious philosophy goals, including 'super stretch' performance levels • Further align executive incentives with long-term stockholder value • Simplified key program elements • Enhance the overall clarity of the program The Board believes these changes strengthen the alignment between pay and performance and support long-term value creation 

COMPARISON OF CEO COMPARISON OF OTHER NEOs COMPENSATION DESIGN COMPENSATION DESIGN 2025 2026 2025 2026 LINK TO PERFORMANCE This combination of metrics (organic Revenue and Performance 50% Organic Revenue; 40% Organic Revenue; EBITDA) emphasizes profitability while driving organic 100% EBITDA 100% EBITDA growth Goals 50% EBITDA 40% EBITDA Eliminated duplication of metrics between the short- TERM and long-term incentive programs - % Individual Increased the weighting of objective financial metrics to 0% 0% 35% - 50% 20% SHORT INCENTIVE Performance better align with stockholder interests Performance Established symmetrical ranges above and below target Asymmetrical Symmetrical Asymmetrical Symmetrical Slope for threshold and maximum performance goals Three-Year Performance 40% 80% 40% 60% Conditioned majority of NEO pay on performance Stock Awards Simplified plan design for greater transparency and Mix Time Based- clarity - 20% - 40% RSUs Time-based RSUs are smaller proportion of the total equity award Performance Instrument Based 45% Eliminate 45% Eliminate Simplified plan design for greater transparency Restricted INCENTIVE Stock Awards TERM Stock Options 15% Eliminate 15% Eliminate Simplified plan design for greater transparency - 75% Adjusted EPS¹; 75% Adjusted EPS¹; Use of rTSR and Adjusted EPS fosters alignment Three-year Cumulative Three-year Cumulative LONG Performance 25% Relative TSR² 25% Relative TSR² between plan payouts, stockholder experience, and Revenue; Relative TSR Revenue; Relative TSR Goals (Three-Yr Performance (Three-Yr Performance aligns with market practice Modifier ±20% Modifier ± 20% Period) Period) rTSR metric rewards top-decile performance Performance Established symmetrical ranges above and below target Asymmetrical Symmetrical Asymmetrical Symmetrical Slope for our threshold and maximum performance goals Performance Award Design Increased rTSR target to 55th percentile, with threshold Relative TSR of 30th percentile 50th Percentile 55th Percentile 50th Percentile 55th Percentile Target rTSR payout is capped at 100% if absolute TSR is negative (1) Adjusted EPS= Non-GAAP Net Income per Share - Diluted. (2) Relative TSR payout capped at 100% if absolute TSR is negative.