Who issues the permit in Unincorporated Washington County?

We have not yet confirmed which office issues building permits in Unincorporated Washington County, or whether one does. Ask the Washington County Commission, which is the county's main government office, who handles permits for your address. Your mailing city is not always the permitting authority.

How do I find out the exact requirements for my project?

Run the free PermitRoadmap check for your specific project and answers. It returns the likely permits, documents, inspections, and next steps, each linked to its official source.

Do I need a permit for accessory dwelling unit (adu) in Unincorporated Washington County?

We have not yet confirmed which office issues building permits in Unincorporated Washington County. Ask the Washington County Commission who handles permits for your address before you start.

The requirements below come from Missouri rules. They are not a statement that Unincorporated Washington County issues or does not issue permits.

Permit likely required

Most likely yes. Accessory dwelling unit (adu) usually requires a permit in Missouri. Confirm the specifics with Unincorporated Washington County.

Missouri has no statewide ADU law; whether an ADU is allowed, and its size and placement, are set by local zoning where it exists. Missouri has no statewide residential building code; whether a permit is required depends entirely on the city or county code that covers the address. Sewage from a new dwelling must go to a public sewer or an on-site system meeting the state standard, which the Department of Health and Senior Services enforces where the city or county has not adopted it; some large rural lots are excluded (RSMo 701.031 and 701.040). Plumbing, electrical, and mechanical permits for the ADU come from the city or county code where one exists; in cities of 15,000 or more, state law directs the city to require a plumbing permit (RSMo 341.060). A new accessory dwelling unit, detached or added on, is new residential construction. It normally needs a building permit with full plans, plus separate electrical, plumbing and mechanical permits, and zoning approval for a second unit on the lot. Utility connections (water, sewer or septic, and electric service) are usually reviewed as part of the permit, so ask about capacity and connection fees early.

Get your exact answer

Answer a few questions to see the specific permits, documents, and next steps for your accessory dwelling unit (adu) in Unincorporated Washington County.

Check my project

Documents you may need

Building permit application to the city or county building office, where a local code appliesLikely required
Zoning approval confirming an ADU is allowed on the lot, where the city or county has zoningLikely required
Site plan with setbacks, and construction drawings with a floor planLikely required
Electrical, plumbing, and mechanical permits, where the local code requires themLikely required
On-site sewage (septic) system approval if the ADU is not served by a public sewerSometimes required

Possible inspections

Footing and foundation inspections, where a local code applies
Framing, electrical, plumbing, and mechanical rough-in inspections, where a local code applies
Septic system inspection where the county or the Department of Health and Senior Services requires it
Final inspection before occupancy, where a local code applies

Frequently asked questions

Do I need a permit for accessory dwelling unit (adu) in Unincorporated Washington County?: Most likely yes. Accessory dwelling unit (adu) usually requires a permit in Missouri. Confirm the specifics with Unincorporated Washington County. The documents and inspections sections on this page list what the building department typically asks for.
Who issues the permit in Unincorporated Washington County?: We have not yet confirmed which office issues building permits in Unincorporated Washington County, or whether one does. Ask the Washington County Commission, which is the county's main government office, who handles permits for your address. Your mailing city is not always the permitting authority.
How do I find out the exact requirements for my project?: Run the free PermitRoadmap check for your specific project and answers. It returns the likely permits, documents, inspections, and next steps, each linked to its official source.

PermitRoadmap is an independent informational service and is not affiliated with any city, county, state agency, or permitting authority. Results are informational and do not replace confirming requirements with the local permitting authority.