What's Required in St. Louis
- One attached or detached ADU per parcel, permitted by right in all residential zoning districts (Ord. 72036, Zoning Code 26.20.021)
- Area up to 50% of the largest primary dwelling unit or 750 sq ft, whichever is greater, and never more than 1,200 sq ft; height not over 25 ft
- Not beyond the average front yard line; at least 3 ft from any side lot line; detached units at least 10 ft from the rear of the dwelling
- No off-street parking required for the ADU, but the primary dwelling's required parking must be kept or replaced
- Building permit: $25 application plus $10 per $1,000 of estimated construction cost or fraction thereof over $3,000
- Electrical permit for new construction ($25 + $120) by a city-licensed electrical contractor; plumbing ($30 + $10 per fixture + inspections) by a city-certified plumber; mechanical ($25 + $40) by a city-licensed mechanical contractor
- Plans: five sets or online upload; a new dwelling usually needs an architect's or engineer's seal; a Zoning Only application can test the site first
- Certificate of Inspection ($120) before a vacant residential unit is occupied; final building inspection before occupancy
- Cultural Resources Office review in a City Historic District
Can You Build an ADU in St. Louis?
Yes, since September 29, 2025. Ordinance 72036 amended the Zoning Code to define attached and detached accessory dwelling units and permit them by right in all residential zoning districts; the site rules sit in Section 26.20.021 and apply to the A district and, because the code is cumulative, to the B, C, D and E dwelling districts. A detached ADU is an independent dwelling unit on the same parcel as a single-family, two-family or semi-detached two-family dwelling, separate from the principal structure; an attached ADU is connected to but not within the house. Interior or basement apartments are not ADUs under the code; the Planning agency says they are regulated as an additional dwelling unit instead. Duplex owners can add one.
St. Louis ADU Rules by Scenario
| What you are doing | City filing | Rule |
|---|---|---|
| New detached backyard ADU costing $150,000 | Building permit $1,525 ($25 + $1,500) plus trade permits about $350 | Fee table; electrical, plumbing and mechanical schedules |
| ADU built into or above an existing garage, $80,000 | Building permit $825 plus trade permits about $350 | 26.20.021 B.2 allows ADUs on nonconforming garages |
| Attached ADU with its own entrance | Building permit plus trade permits; at least 3 ft from side lot lines | 26.20.021 A.5 |
| ADU over 1,200 sq ft, taller than 25 ft, or a second ADU on the parcel | Not allowed | 26.20.021 A.2, A.3, A.4 |
| Apartment inside the existing house or basement | Not an ADU; reviewed as an additional dwelling unit under density rules | Planning ADU page |
| Detached ADU closer than 10 ft to the rear of the house, or in front of the house | Not allowed | 26.20.021 A.5, B.1 |
| Moving in or renting the finished unit | Certificate of Inspection, $120, before occupancy | Residential occupancy permit page |
What Are the ADU Size and Setback Rules?
Height is capped at 25 feet, garage-top units included. Floor area may be up to 50 percent of the largest primary dwelling unit or 750 square feet, whichever is greater, and never more than 1,200 square feet. Only one ADU per parcel. No ADU may stand beyond the average front yard line, every ADU must be at least 3 feet from any side lot line, and a detached ADU must be at least 10 feet from the rear of the house. No off-street parking is required for the ADU, but it may not remove the primary house's required parking, so an ADU in the A district garage means replacing that space. Building on the foundation of, or above, a nonconforming garage is expressly allowed. Historic districts may add design review through the Cultural Resources Office.
How Much Does an ADU Permit Cost in St. Louis?
The building permit follows the estimated construction cost, which the ordinance defines to include site work, structure, finishes and the plumbing, mechanical and electrical work: $25 application plus $10 per $1,000 or fraction above $3,000. A $150,000 detached unit pays $1,525; an $80,000 garage conversion pays $825. Trade permits are separate: electrical new construction with a new service is $25 + $120 = $145; plumbing is $30 plus $10 per fixture plus $25 rough and $25 finish inspections, about $140 for six fixtures; a furnace and condenser combination is $25 + $40 = $65. That is roughly $350 in trade permits, for totals of about $1,175 and $1,875. Water taps and sewer connections are $25 each on the plumbing permit if the unit gets its own. Before anyone moves in, a Housing Conservation District Certificate of Inspection ($120) is required for a vacant residential unit. The $25 building application fee is paid at filing and the permit fee at issuance.
How Do You Apply for an ADU Permit in St. Louis?
The Planning agency's advice is to treat it like any major construction project: permits come from the Building Division, with a Zoning Section review built into the process. If you are unsure the design fits 26.20.021, file a Zoning Only application first (two sets of plans with site plan, floor plans, use and elevations) so staff can rule on the site before you pay for full drawings. The full application goes online at stlcitypermits.com or to Room 425, City Hall, with five sets of plans. A new dwelling is the kind of project the Plan Exam page says usually needs an architect's or engineer's seal; the ordinance waives seals only for owner-drawn plans on an owner-occupied one- or two-family home, so ask the Hot Spot desk before relying on your own drawings. Routed reviews typically take 2 to 4 weeks. The permit issues in the owner of record's name for six months, extendable in writing, and a final inspection is required before occupancy.
How to Get a ADU Permit in St. Louis
Check the parcel against 26.20.021
One ADU per lot, 1,200 sq ft and 25 ft caps, 3 ft side setback, 10 ft from the house for a detached unit, parking kept. Look up the zoning district on Address Search.
File a Zoning Only application if unsure
Two sets of plans with site plan, floor plans, use and elevations let Zoning staff confirm the site before you pay for full construction drawings.
Prepare sealed construction documents
A new dwelling usually needs an architect's or engineer's seal. Apply online at stlcitypermits.com or at Room 425 with five sets and the $25 application fee.
Pay the building permit fee and pull trade permits
$1,525 on a $150,000 unit at issuance; your electrician, plumber and mechanical contractor pull their own permits ($145, about $140, $65).
Inspect through to occupancy
Call each inspection a working day ahead, pass the building final, then get the $120 Certificate of Inspection before anyone moves in.