ADU Rules in St. Louis, Missouri

St. Louis City vs. St. Louis County — An Important Distinction

The City of St. Louis is an independent city — it has been entirely separate from St. Louis County since 1876 when it was legally detached from the county. This makes St. Louis unusual among U.S. cities: it is its own county-equivalent jurisdiction, governed by its own mayor, aldermanic board, zoning code, and planning department.

If your property is in St. Louis City, this guide applies. If your property is in a municipality within St. Louis County (such as Clayton, Kirkwood, Webster Groves, Florissant, Chesterfield, etc.), that municipality's own zoning code governs — not the City of St. Louis.

Missouri State Law

Missouri has enacted no statewide ADU preemption law. Unlike California or several other states, Missouri does not mandate that cities allow ADUs or override local zoning standards. All ADU rules in St. Louis City flow from the city's own Zoning Code.

Permitted ADU Types

Within the City of St. Louis, ADUs may be permitted in residential zoning districts. Common forms include:

• Carriage House / Detached Accessory Structure: St. Louis has a strong tradition of carriage houses and rear-alley structures, many of which are historic. Converting an existing carriage house to an ADU may be feasible in appropriate districts.
• Attached Addition: An addition to the primary structure containing a separate dwelling unit.
• Interior Conversion: Converting existing space (basement, upper floor) within the primary structure into a separate unit.
• Two-Family Conversion: Some districts permit two-family use, which may accommodate an ADU arrangement.

Permitted ADU types and applicable standards differ by zoning district. St. Louis's residential zones range from single-family (A and B districts) through multi-family designations. Confirm your district with the Planning and Urban Design Agency.

Size, Setbacks, and Height

Dimensional standards are set by the applicable zoning district. General reference points:

• Setbacks: Rear yard and side yard setbacks for accessory structures are district-specific; historic carriage house structures may have pre-existing nonconforming setbacks.
• Height: Accessory structure height limits apply and vary by district.
• Lot coverage: Maximum lot coverage limits affect what can be built in addition to the primary structure.

Always verify current standards with the city's Planning and Urban Design Agency.

Historic Districts and Preservation Overlays

St. Louis has numerous local historic districts — including Lafayette Square, Soulard, Cherokee Street area, Compton Hill, and many others — as well as properties listed on the National Register of Historic Places. If your property is in a locally designated historic district, any ADU project must go through the Cultural Resources Office for review. Design standards may limit exterior alterations.

Mississippi River and Floodplain

Parts of the City of St. Louis lie adjacent to the Mississippi River. Properties in floodplain areas must comply with FEMA NFIP requirements and city floodplain management ordinances. Check FEMA's Flood Map Service Center for your property's flood zone designation before planning any ADU.

Permit Process

1. Confirm zoning district: Look up your property on the St. Louis City zoning map at stlouis-mo.gov.
2. Check for overlays: Determine if your property is in a historic district, floodplain, or other overlay zone.
3. Pre-application consultation: Contact the Planning and Urban Design Agency to discuss your project before submitting plans.
4. Building permit application: Submit plans to the Building Division of the Department of Public Safety. Plans must include site plan, floor plans, and elevations.
5. Zoning review: Planning staff reviews for zoning compliance; historic review if applicable.
6. Permit issuance and construction: Obtain permit before beginning work.
7. Inspections and Certificate of Occupancy: Complete required inspections and obtain a CO before occupying the unit.

Contact Information

City of St. Louis Planning and Urban Design Agency 1015 Locust Street, Suite 1200, St. Louis, MO 63101 Phone: (314) 622-3400 Website: stlouis-mo.gov/government/departments/planning/

Cultural Resources Office (Historic Preservation) 1015 Locust Street, St. Louis, MO 63101 Phone: (314) 622-3400

Disclaimer: Zoning regulations change frequently. Always verify current ADU rules directly with the City of St. Louis Planning and Urban Design Agency before beginning any project. This page is for informational purposes only and does not constitute legal or zoning advice.