The purpose of a Certificate of Appropriateness is to help maintain and return the lost or diminished architectural significance of Houston's tangible past. A Certificate of Appropriateness is required for projects that would alter the exterior appearance of a designated historic property. This includes alterations, additions, demolition, or relocation of a landmark or structure within an historic district or new construction within an historic district. The Historic Preservation Ordinance does not regulate land use, exterior paint colors, interior alterations or landscaping.

A Certificate of Appropriateness ("C of A") is needed when the proposed work requires a Department of Buildings permit and will affect significant protected architectural features. Additions, demolitions, new construction, and removal of architectural features such as stoops and cornices usually require a C of A. When the proposed work does not require a permit from the Department of Buildings, but has been denied a PMW by the Commission, the owner may apply for a Certificate of Appropriateness.

The Landmarks Law requires that a public hearing be held for each C of A application. At least six Commissioners must vote in favor of an application in order for it to be approved. The Landmarks Commission conducts C of A hearings every month, usually on the fourth Tuesday of the month. Notice of each Landmarks Commission hearing is published in the City Record for ten days before the hearing, and Community Boards are notified of applications affecting properties within their districts.

At the hearing, the applicant is given the opportunity to explain why he or she believes that the application should be approved. The public may also comment on the proposed work. The applicant and the public may also submit written statements before the hearing. In many cases, the Commissioners discuss and vote on a C of A application on the same day that the public hearing is held. Sometimes, however, they request additional information about the proposal before voting. After the public hearing is closed, the applicant and the public may submit additional written statements about the proposal up until the time when the Commissioners vote on the application.

Usually the Commissioners will make a decision on a C of A application based on "presentation" or "design development" drawings of a proposed project. After the Commissioners vote to approve an application, the applicant may be required to submit construction drawings in order for the approval to become final. The actual C of A permit is not issued until the staff has reviewed the final construction drawings to make sure that the final plans are consistent with the proposal approved by the Commissioners.