Accessory dwelling unit cost — St. Louis, MO

Last updated: September 24, 2026

What affects the price

Building an ADU involves several moving parts that shape your total investment. The final figure depends heavily on whether you choose a prefab unit or a custom build from the ground up. Site conditions are another major factor; if your property requires extensive grading, utility trenching, or foundation work, costs will climb. You also have to consider your choice of finishes, like high-end appliances or custom cabinetry. Navigating local permit fees and impact assessments also adds to the total.

About this service

Total costs for an ADU vary based on the size of the unit, the complexity of the site, and the quality of materials you select for the kitchen and bathroom. Things like trenching for new sewer lines or upgrading your main electrical panel can also impact the bottom line. We provide an exact pricing confirmed free on the call.

Frequently asked in St. Louis

Also serving nearby: O Fallon · Springfield · Columbia · Champaign · Peoria

All cities in Missouri →

More on this: HUD ADU resources — ADU policy and financing research.

Service area details

Serving all of St. Louis — population 281,754; about 4,565 residents per square mile across 61.7 sq mi.

Neighborhoods served in St. Louis: Central West End, Tower Grove South, Soulard, The Hill, Benton Park — and every surrounding area.