Can I Build ADU in St. Louis, MO? (2026)
Find out if you can build a adu in St. Louis, MO. Rules, permit requirements, costs, and next steps.
Yes, ADUs are allowed in St. Louis, MO. California state law ensures by-right approval.
An Accessory Dwelling Unit (ADU) is a secondary housing unit on a residential lot — also called a granny flat, in-law suite, or backyard cottage. In St. Louis, MO, ADUs are permitted by right under state law, making them one of the most accessible ways to add housing on your property. ADUs can be detached, attached to the main home, or created by converting an existing garage or accessory structure.
ADU Rules in St. Louis
ADU Rules in St. Louis: Detached ADU max size: 900 sq ft. Attached ADU max size: 700. Setbacks: 3 ft side, 3 ft rear. Owner-occupancy: No state-mandated owner-occupancy requirement. St. Louis City regulations do not impose owner-occupancy for ADU approval..
Costs & Fees
ADU permits in St. Louis: Total estimated permit cost: $1,000-$4,500 total permit fees. Contact the St. Louis Building Division (Community Development Administration) for current fee schedule.
Timeline
St. Louis Building Division permit review: typically 4-8 weeks. Construction 3-9 months. The St. Louis Vacancy Collaborative and New Roots program may offer expedited review for eligible properties.
Next Steps
Next steps for building an ADU in St. Louis: 1. Review full ADU rules including size limits, setbacks, and parking. 2. Apply for an ADU permit through the St. Louis Building Division (Community Development Administration) at https://stlouis-mo.gov/government/departments/building/permits.cfm. 3. Hire a licensed contractor familiar with MO ADU requirements.
St. Louis Building Division (Community Development Administration)
Phone: (314) 622-3313
Website: https://stlouis-mo.gov/government/departments/building
Online Permits: https://stlouis-mo.gov/government/departments/building/permits.cfm
Frequently Asked Questions
Is St. Louis City the same as St. Louis County?
No — and this is a critical distinction. St. Louis City is an independent city that separated from St. Louis County in 1876. The City is not part of any county and functions as its own county-equivalent. When searching for zoning, permit, or land use information, make sure you are looking at St. Louis City resources (stlouis-mo.gov), not St. Louis County resources (stlouisco.com). Properties within city limits use the City's zoning code; properties in the suburbs use St. Louis County or their respective municipality's codes.
Can I build an ADU on my St. Louis rowhouse lot?
Yes. St. Louis allows ADUs in residential zones. The traditional St. Louis rowhouse lot typically has a rear yard with alley access — the natural location for a detached ADU or carriage house conversion. No replacement parking is required. Contact the St. Louis Building Division for current requirements and the Planning and Urban Design Agency for zoning questions. If your home is in a local historic district (Soulard, Lafayette Square, etc.), you'll need a Certificate of Appropriateness before building.
What is the St. Louis City home business permit process?
St. Louis does not require a separate home occupation permit, but home businesses must comply with zoning code restrictions: no customers visiting the premises in residential zones A-E, no exterior signage, no non-resident employees, and no more than 25% of floor area used for business. If you plan to operate a business requiring a state or city license (e.g., daycare, food service, professional services), contact the St. Louis License Collector for applicable license requirements.
How does St. Louis handle short-term rentals like Airbnb?
St. Louis requires an STR license ($200/year) for rentals of less than 31 consecutive days. Both hosted (room rental while owner is present) and whole-home rentals may be allowed depending on the zone. All listings must include the license number. St. Louis imposes a Hotel/Motel Guest Tax on STR revenues in addition to Missouri state sales tax. Contact the St. Louis License Collector at (314) 622-4111 for current licensing requirements.
My home is in the Soulard Historic District — what does that mean for renovations?
Soulard is one of St. Louis's most celebrated historic neighborhoods, designated as both a local historic district and a National Register Historic District. For exterior changes (including additions, window replacements, siding, fences, roofing material changes, and ADU construction visible from public right-of-way), you must obtain a Certificate of Appropriateness from the St. Louis Preservation Board before beginning work. Interior changes do not require Preservation Board review. Applications are reviewed at monthly Preservation Board meetings. Contact the Cultural Resources Office at (314) 622-3400.
Source: St. Louis Building Division (Community Development Administration) — Building Regulations. Last verified April 6, 2026. View source