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
The St. Louis County Zoning Ordinance controls second dwellings through its definition of family in §1003.020. A family is one individual, or two or more persons related by blood or marriage, or a group of not more than three unrelated persons, living together and subsisting in common as a single non-profit housekeeping unit using not more than two kitchens. Those two limits, three unrelated people and two kitchens, are what a basement or backyard apartment usually runs into.
Accessory Dwellings & Family Definition
Heavy RestrictionsSt. Louis County Code §1003.020
Sec. 1003.020. — St. Louis County's Zoning Ordinance defines a family as an individual or two (2) or more persons related by blood or marriage or a group of not more than three (3) persons who need not be related by blood or marriage living together and subsisting in common as a single non-profit housekeeping unit using not more than two (2) kitchens.
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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