St. Louis County, MO Accessory Structures: ADU Rules (2026)

St. Louis County Charter authority to zone is exercised through the County Zoning Ordinance, codified at Title X, Chapter 1003 SLCRO 1974, as amended. The zoning ordinance applies only to unincorporated St. Louis County. In incorporated cities (Florissant, Kirkwood, Ferguson, Chesterfield, Maryland Heights, and 80+ others), each municipality enforces its own zoning code. Under the County Zoning Ordinance, accessory buildings, structures and uses are permitted only when they are customarily found in conjunction with the primary use, are reasonably necessary to it, are clearly subordinate to it, and serve only to further the successful utilization of the primary use. Detached above-ground accessory structures may not cover more than seven percent of the lot area in aggregate. A single-family lot is permitted one detached garage plus up to two additional detached accessory structures (three total). Individual structures are limited to 400 square feet or 50 percent of the primary structure's building footprint, whichever is greater, and aggregate accessory floor area may not exceed 1,000 square feet regardless of primary structure size. No accessory building is permitted in the front yard, and no accessory building may be taller than the primary structure. Standard residential setbacks apply (typically 20 ft front, 6 ft side, 15 ft rear in R-4). The ordinance does not generally authorize separate detached dwelling units (true ADUs) as accessory uses in standard residential districts. Property owners seeking a separate dwelling unit typically need a zoning amendment, conditional use permit, or must rely on internal conversions where allowed by code. Permits are issued by the St. Louis County Department of Public Works.