Can I Build ADU in St. Louis, MO? (2026)

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 sq ft. 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)

Frequently Asked Questions