Accessory Structures in St. Louis County, MO (2026)
2 county-level rules, plus city-specific rules for 1 city in St. Louis County, Missouri.
Verified from official government sources
ADU Rules
St. Louis County's zoning code never defines an accessory dwelling unit. Section 1003.020 supplies the building blocks instead: an accessory structure is incidental to the principal use, an apartment is any space with separate cooking facilities, and a family uses not more than two kitchens. In the R-1 district, all detached accessory structures together are capped at 1,000 square feet.
St. Louis County Accessory Dwelling Rules
Some RestrictionsSt. Louis County Code § 1003.020
Accessory structure: Any structure, the use of which is incidental to the principal use of another structure on the same premises. ... Apartment: A room or suite of rooms within a building, provided with separate cooking facilities and intended as a single dwelling unit. ... Dwelling: Any building, or portion thereof, which is designed or used exclusively for human habitation, except hotels, mo...
Shed & Carport Rules
A detached, one-story storage shed, tool shed, or playhouse in unincorporated St. Louis County needs no building permit as long as its floor area stays at or under 120 square feet under the county's amended IBC Section 105.2; go over that, and the full building-permit process applies.
St. Louis County Shed Permit Size Limit
Some RestrictionsSt. Louis County Code § 1115.101 (amended IBC § 105.2, Item 3); § 1003.107
105.2 Work exempt from permit. Exemptions from permit requirements of this code shall not be deemed to grant authorization for any work to be done in any manner in violation of the provisions of this code or any other laws or ordinances of this jurisdiction. Permits shall not be required for the following:...3.One-story detached accessory structures used as tool and storage sheds, playhouses an...
Accessory Structures by City in St. Louis County
1 cities in St. Louis County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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