St. Louis County, MO Accessory Structures: ADU Rules (2026)
ADU rules in St. Louis County, MO, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Defined ADU term
- None in Chapter 1003
- Kitchens per family
- Not more than 2
- Unrelated persons per family
- Not more than 3
- R-1 accessory structure total
- 1,000 sq ft gross floor area
- R-1 lot coverage limit
- 7% of lot area
- R-1 detached garages
- 1 maximum
- Maximum penalty
- $1,000 fine or 6 months jail, or both
Summary
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.
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, motels, or house trailers. ... Dwelling unit: A room or group of rooms located within a dwelling forming a habitable unit for one (1) family. ... Dwelling, single family: A building designed for or occupied exclusively by one (1) family, excluding earth sheltered dwellings. ... Family: 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 utilizing not more than two (2) kitchens.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 85 | Traffic: Supplement 80).
Full Breakdown
A search of Chapter 1003 finds no definition for "accessory dwelling unit," "guest house" or "in-law suite," so an ADU question is answered by combining the definitions in Section 1003.020 with the district rules. An accessory structure is any structure whose use is incidental to the principal use of another structure on the same premises. The residential examples are storage structures, gazebos, structural trellises and storage sheds. Playsets and in-ground pools are excluded, and above-ground pools are included. A detached accessory structure shares no common walls or roof with the primary building, while an attached structure shares at least one wall and the roof.
The dwelling definitions decide whether a building is an ADU in substance. A dwelling is any building or portion of one designed or used exclusively for human habitation, other than hotels, motels or house trailers. An apartment is a room or suite of rooms with separate cooking facilities, intended as a single dwelling unit. A dwelling unit is a habitable unit for one family, and a single family dwelling is a building occupied exclusively by one family. A two family dwelling is built for or occupied exclusively by two families, and a three family dwelling by three.
The family definition carries the most weight. A family is an individual, people related by blood or marriage, or a group of not more than three unrelated persons living together as a single non-profit housekeeping unit using not more than two kitchens. A second kitchen inside a house therefore fits one family. A third kitchen, or a second household, moves the building away from the single family definition.
Under the R-1 residence district, Section 1003.111 permits single family dwellings and sets the limits for detached accessory structures. Together they can cover no more than 7 percent of the lot. Each can be 400 square feet or 50 percent of the primary building footprint, whichever is greater, and the aggregate cannot pass 1,000 square feet of gross floor area. A lot can hold one detached garage and two other detached accessory structures. No accessory building can sit in the front yard or stand taller than the primary structure. Section 1003.170 also bars adding dwelling units to a nonconforming residential structure.
Violations & Fines
The Director of Transportation and Public Works enforces Chapter 1003 under Section 1003.200. The Director can post stop-work and stop-use orders, withhold or revoke permits, and refer a violation to the County Counselor. The County can seek an injunction or an abatement order, with its costs assessed against the property. A conviction carries a fine up to $1,000 per offense, up to six months in the County Jail, or both. Each day after written notice is a separate offense, and owners, contractors and tenants can be held jointly responsible.
Frequently Asked Questions
Does St. Louis County have a specific ADU ordinance?
How many kitchens can a single family home have?
How large can a detached accessory building be in the R-1 district?
What penalties apply to a zoning violation in St. Louis County?
Sources & Official References
Other rules in St. Louis County
How St. Louis County compares: Easiest Cities to Build an ADU·Compare St. Louis County to another location·View the Missouri accessory structures overview
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