Washington County, Missouri Building Permit Guide

Washington County government in Potosi has no planning, zoning or building department, so a house or shed on rural land needs no county building permit. The permits that do apply outside the towns are a septic permit from the Washington County Health Department (573-438-2164), currently $125, and a floodplain permit where a site lies in a mapped flood zone. Potosi’s building inspector ($150 for a new home) and Caledonia’s village clerk issue building permits in those towns.

Washington County building department: who handles permitting

The county’s offices are the elected offices plus the airport, emergency management, the industrial development authority and the juvenile office; none of them reviews building plans. The County Commission meets at the courthouse, 102 N. Missouri St., Potosi (commission office 573-436-7704, 8:00 a.m. to 4:30 p.m.). On April 28, 2026 the Commission met with the Meramec Regional Planning Commission, the regional planning agency, to talk about county planning.

Potosi building permit fees

Potosi’s building permit fee sheet, dated August 9, 2022, sets flat amounts for most homeowner work. The City also needs a plot plan on the back of the application before it issues a permit, and the builder’s business licence number goes on the form.

The City’s code (Chapter 5, as compiled in July 2023) sets plan examination fees on the same value bands for one- and two-family homes and “the greater of $500.00 or 8.5 cents per square foot” for commercial plan review. Before a vacant home is occupied or a rental changes hands, the building inspector inspects it for a residential occupancy permit; the fee sheet lists $30 for that permit, while the code text still says $15.

Septic and flood rules on rural land

Septic systems: the Washington County Health Department at 520 Purcell Drive, Potosi, enforces the county’s on-site wastewater ordinance of May 24, 1999, which follows state law and covers the unincorporated areas; inside the four towns, the state health department handles septic permits. A construction permit is always needed for a system that serves a business, church, club, government building or more than one home, and for any system on a property under 3 acres. A single home on 3 acres or more is excluded only while every part of the system stays more than 10 ft from the property line and no sewage leaves the lot or pollutes water; a Notice of Sewage Violation cancels the exclusion.

Replacing only a septic tank needs no soil evaluation: a department inspector visits first to check that the drain field is not failing.

Floodplains: Washington County, Potosi, Caledonia, Irondale and Mineral Point all take part in the National Flood Insurance Program, with flood maps dated June 5, 2020. The State Emergency Management Agency lists the presiding commissioner at the courthouse (573-438-6111) as the floodplain administrator for unincorporated land; check the map before any grading or building on rural land.

Building on rural land in Washington County

Missouri building code

Missouri has no mandatory statewide building code for private construction: a county, fire protection district or municipality may adopt one (usually by reference under RSMo 67.280), and in parts of the state none has. Building codes in Missouri

Washington County permit FAQs

Do I need a county building permit in rural Washington County, Missouri?

No. The county has no building, planning or zoning department; the county-level approvals are a septic permit and, in flood zones, a floodplain permit.

When is a septic permit required?

For any system on a property under 3 acres, for systems serving a business, church, government building or several homes, and on larger lots that do not meet the state exclusion conditions. Outside the towns the Health Department issues it for $125; inside the towns the state health department handles septic permits.

How much is a building permit in Potosi?

$150 for a new home under the City’s August 2022 fee sheet; sheds, decks and carports are $30 each. Call the City at 573-438-2767.

Is the City of Washington in this county?

No. The City of Washington is in Franklin County. Washington County’s county seat is Potosi.

Who handles floodplain questions?

The county floodplain administrator listed by the State Emergency Management Agency is the presiding commissioner at the courthouse in Potosi (573-438-6111); each town has its own contact.

Washington County permit search and official resources

• Washington County Commissioners and office hours
• Washington County Health Department septic (OWTS) information
• Washington County septic construction permit requirements
• Washington County septic permit application and instructions (2026)
• Missouri DHSS onsite wastewater permit authority by county
• SEMA emergency and floodplain contacts, Washington County
• FEMA Community Status Book: Missouri
• City of Potosi government, permits and phone numbers
• City of Potosi building permit fees (August 2022)
• City of Potosi building permit application
• Village of Caledonia Code of Ordinances (scanned PDF)

Simplify Washington County permitting with Alliance Permitting

Alliance Permitting is a permit documentation and submission company in Missouri. We prepare your paperwork, file it correctly and coordinate with the permitting authority through issuance. We are not a contractor and we do not perform licensed plan review or inspections; that work stays with your licensed team and the jurisdiction.

This guide is provided by Alliance Permitting for general information about Washington County permitting. Requirements, fees, and procedures change; confirm details with the permitting authority before relying on them.