Property owners and their design professionals (architects, builders, etc.) should consult these design guidelines as early as possible when planning a project that involves a change to the exterior of a building, including an addition, or the construction of a new building within these historic districts. The City’s Historic Preservation staff in the Planning and Development Department and the Houston Archaeological and Historical Commission (HAHC) will also use these design guidelines to determine whether to approve an application for a Certificate of Appropriateness (COA) for a project that proposes to make changes to a building in these historic districts.