What's Required in St. Louis
- Building permit required for all secondary dwelling units.
- Must have full kitchen and bathroom facilities for habitation.
- Requires professional architectural plans including foundation.
- Must meet lot coverage and setback rules for the zoning district.
- Owner-occupancy of the primary home is often required.
- Maximum size capped by specific neighborhood zoning.
How Much Does an ADU Permit Cost in St. Louis?
Budget $1,000–$3,500 for the St. Louis permit itself (valuation-based), issued by the Building Division — but treat that as the entry fee, not the total. ADUs are full dwelling units, so the permit stack usually includes building, electrical, plumbing, and mechanical trade permits, plus plan review. One local note: includes building, zoning, and utility capacity fees. Get an itemized fee estimate from the department before finalizing your budget; the spread between a simple conversion and a new detached build is wide.
ADU Size Limits and Zoning Rules in St. Louis
Before drawing plans, check the two gates that sink most ADU projects: size and zoning. Size ceilings vary by lot and ADU type (attached, detached, or conversion), so confirm your specific configuration. Other recorded conditions: owner-occupancy of the primary home is often required. Setbacks, height limits, and utility connections layer on top of those rules. The Building Division can confirm what your specific lot allows — worth doing before you pay for architectural drawings rather than after.
How to Get a ADU Permit in St. Louis
Step 1: Zoning Verification
Confirm an ADU is allowed on your lot via the Building Division.
Step 2: Design Documents
Prepare detailed site plans and architectural drawings.
Step 3: Submit Application
Apply for a New Accessory Structure permit online.
Step 4: Multi-Agency Review
Checked by Building, Zoning, and Fire departments.
Step 5: Inspections
Full series from footings to final occupancy sign-off.