Olivette

St. Louis County, MO (987,000 residents in suburban St. Louis - distinct from the City of St. Louis which is a separate independent jurisdiction) administers the St. Louis County Zoning Ordinance for unincorporated territory. The county includes 88 incorporated municipalities (a notably high number); each sets its own ADU rules. Unincorporated pockets are scattered throughout.

• St. Louis County Zoning Ordinance (St. Louis County Revised Ordinances)

State-floor overlay: Missouri has not enacted statewide ADU preemption.

Saint Louis County administers flood-hazard, and (where mapped) coastal, wildland-fire, historic, and airport overlays that shape ADU project feasibility. The most consistent overlay across the county is FEMA NFIP floodplain regulation; other overlays apply to specific geographies inside the county.

• FEMA NFIP Special Flood Hazard Areas in Saint Louis County — A new ADU in a mapped SFHA must be elevated to or above the Base Flood Elevation; cost impact on the project is often material.
• Historic districts and individually-listed historic resources

Saint Louis County issues building permits for parcels in unincorporated territory through its development services / planning department, with separate review tracks for zoning conformance, building-code compliance, on-site sewage where applicable, floodplain compliance, and addressing. Inside incorporated municipalities, city departments handle their own permits; the county's authority is geographically limited to unincorporated territory. An ADU permit application is typically processed as a residential building permit with a zoning verification step against the county's ordinance for the parcel's zoning district.