St. Louis has made it notably easier to build an accessory dwelling unit in recent years, but the permit process still requires careful preparation. An ADU (accessory dwelling unit) is a secondary housing unit on the same lot as a primary residence. These units go by many names: granny flat, in-law suite, backyard cottage, carriage house. Landlords build them to generate rental income, house family members, or increase long-term property value. If you're planning one in the city, knowing exactly which department to contact, what paperwork to file, and where projects stall will save you real time and money.
Types of ADUs allowed in St. Louis
St. Louis permits three main ADU configurations under its current zoning code:
- Detached ADUs: Standalone structures built separately from the main house, often called carriage houses or backyard cottages. These offer the most privacy but require the most construction work.
- Attached ADUs: Additions built onto the side or rear of the primary home. They share a wall with the main structure but have a separate entrance.
- Interior conversions: Basement apartments and garage conversions that repurpose existing square footage. These tend to cost less since the shell already exists, though the finished space must meet all building code requirements for habitable use.
Junior ADUs (JADUs), which are common under California law, don't have a formal equivalent in St. Louis's code. If your unit is within the main home's footprint, it will generally be treated as an interior conversion ADU.
Zoning rules that shape your ADU plan
The city's most significant shift came with Board Bill 43 (2023), which now allows ADUs by right in all residentially zoned districts. That removes the old requirement to seek special use approval in many neighborhoods, a step that previously added months to the process.
Even so, the rules have real teeth. Key requirements include:
- Your lot must be at least 50 feet wide and cover a minimum of 5,000 square feet.
- The ADU must have its own kitchen and bathroom to qualify as a separate dwelling.
- Detached ADUs must respect rear and side setbacks, which vary by zoning district.
- The unit's design must be compatible with the primary residence in scale and materials.
- Lots with multi-family buildings are generally excluded unless the specific district makes an exception.
- Properties in historic districts may face additional design review.
If you're unsure of your zoning classification, the St. Louis City Planning and Urban Design Agency can confirm it quickly.
Owner-occupancy and rental rules
St. Louis generally requires owner-occupancy: either you live in the main house or in the ADU itself. This rule is worth verifying for your specific district, since the 2023 update introduced some variation by zone.
You can rent your ADU for long-term residential use without a separate business license. You do need to comply with Missouri landlord-tenant law, which governs lease terms, security deposits, and eviction procedures. Short-term rentals (platforms like Airbnb) are treated differently and require their own permit and licensing under city ordinance. Don't assume your long-term rental approval covers short-term use.
Step-by-step: the St. Louis ADU permit process
Step 1: Confirm your lot qualifies
Before you draw up plans or hire a designer, verify that your property meets the baseline lot size and width requirements. Check your zoning classification through the Planning and Urban Design Agency. This step is free and takes little time, but skipping it early is one of the most common reasons projects stall later.
Step 2: Pull your setback requirements
Setbacks determine how far your ADU must sit from property lines, the main house, and the street. These vary by zoning district, so don't rely on a neighbor's numbers. Pull your specific requirements from the city's zoning code before finalizing any design. ADUs are generally placed in the rear or side yard.
Step 3: Prepare your application package
Submit the following to the City of St. Louis Building Division:
- Site plan showing the ADU's location, dimensions, and setbacks
- Architectural drawings (floor plan, elevations, sections)
- Proof of lot size and ownership
- Utility connection plan (water, sewer, electric)
- Completed permit application form
If you're converting an existing structure, such as a basement or garage, you'll also need a structural assessment. The OneSTL ADU toolkit walks through documentation requirements in plain language and is worth bookmarking early.
Submitting an incomplete package is the single biggest source of delays. The Building Division will send the application back for corrections, resetting your place in the review queue. Get every document together before you submit.
Step 4: Submit and pay permit fees
Permit fees in St. Louis are calculated based on project valuation. A modest detached ADU typically runs several hundred dollars in permit fees alone, not counting plan review charges. Contact the Building Division directly for the current fee schedule, since these figures change. Plan review for residential projects generally takes two to six weeks. Complex designs or high-volume periods at the Building Division can push that longer.
Step 5: Inspections and certificate of occupancy
Once your permit is issued and construction begins, the Building Division will inspect the work at key stages: foundation, framing, mechanical, electrical, plumbing, and final. Don't schedule a final inspection until all utilities are connected and the unit is fully finished. The city issues a certificate of occupancy once everything passes. You'll need that document before any tenant can legally move in, and lenders will ask for it if you ever refinance.
Common pitfalls to avoid
A few missteps show up repeatedly in St. Louis ADU projects:
- Starting construction before permits are issued. Unpermitted work can trigger stop-work orders, fines, and mandatory demolition of completed work.
- Using generic ADU plans not tailored to your lot. Setbacks, lot coverage limits, and design compatibility rules are site-specific. A plan that worked on a neighbor's lot may not work on yours.
- Forgetting utility coordination. Connecting water, sewer, and electric to an ADU often involves utility companies and city departments separately. Build this into your timeline early.
- Assuming county rules match city rules. St. Louis City and St. Louis County are separate jurisdictions with different zoning codes. If your property is in unincorporated county territory or a county municipality, the city's 2023 ADU rules don't apply. Contact that jurisdiction's planning department directly.
- Skipping historic district review. If your property is in a designated historic district, design approval from the city's Cultural Resources Office may be required before you can even apply for a building permit.
Costs and financial considerations
Construction costs for a St. Louis ADU vary widely. A garage conversion might run $40,000 to $80,000. A new detached unit can reach $150,000 or more depending on size and finishes. Factor in permit fees, architect or designer fees, utility hookup costs, and any land preparation work.
A rentable ADU can generate meaningful monthly income. St. Louis rental markets vary by neighborhood, but a well-located ADU can offset a significant portion of mortgage costs. That rental income also affects your property's appraised value, which matters when you refinance or sell.
If you're renting the unit, carrying landlord insurance in Missouri protects you against tenant-related liability, property damage, and lost rental income. Standard homeowner's policies typically don't cover rental activity, so don't assume your existing coverage is enough.
HOA considerations
Homeowners associations add another layer to the approval process. Even if the city permits your ADU by right, an HOA with covenants covering your property can restrict or prohibit secondary structures, exterior modifications, or rental activity. Review your HOA's covenants, conditions, and restrictions (CC&Rs) before spending money on design. Some HOAs have amended their rules in response to the 2023 zoning changes; others haven't. Don't assume city approval settles the question.
Frequently asked questions
Does St. Louis require owner-occupancy for ADUs?
Generally yes. The owner must typically occupy either the main house or the ADU. The 2023 zoning update introduced some variation by district, so confirm your specific requirement with the Planning and Urban Design Agency before finalizing plans.
How long does ADU permit approval take in St. Louis?
Plan review for straightforward residential projects usually takes two to six weeks. More complex designs or backlogged periods at the Building Division can extend that timeline. Submitting a complete application package the first time is the best way to avoid delays from resubmission requests.
What is the maximum size for an ADU in St. Louis?
The city's zoning code ties ADU size to the proportions of the primary dwelling rather than setting a single square-footage cap. Confirm the specific calculation with the Building Division before finalizing your design.
Can I convert my garage into an ADU in St. Louis?
Yes. Garage conversions are permitted, but the finished space must meet building code requirements for habitable units, including minimum ceiling height, egress windows, insulation, and separate kitchen and bathroom facilities.
Do I need a separate utility meter for my ADU?
Not necessarily. ADUs can share utilities with the primary residence or have independent connections. The choice affects billing and tenant arrangements, so discuss options with your utility providers and contractor during the planning phase.
Are ADUs allowed in St. Louis County as well as the city?
St. Louis City and St. Louis County are separate jurisdictions with different zoning codes. If your property is in unincorporated St. Louis County or a county municipality, contact that jurisdiction's planning department directly. The city's 2023 ADU rules don't extend to county territory.
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