Building permits, zoning and septic rules in Mason County, Washington

Yes. Mason County Community Development requires a building permit for residential construction and enforces the Washington State Building Code on every unincorporated parcel in the county¹. Washington law requires every county to enforce the state building code within its jurisdiction, and a county without its own building department has to contract with another county, city or approved inspection agency; Mason County runs its own¹⁰. State rules let a county adopt a narrow exemption for very small projects, under $1,500 of value in any twelve-month period, that don't touch structural components, egress, light or ventilation, but this never covers a new house and doesn't apply to electrical, plumbing or mechanical work¹¹. Washington's contractor-registration law exempts a person working on their own property or personal residence, but not when the work on their own property is for sale, demolition or lease⁵. That owner-builder exemption does not remove Mason County's building-permit requirement¹.

Mason County Community Development's Building Division issues residential building permits and inspections out of the Community Development office at 615 W Alder St in Shelton¹². The division runs an online SmartGov portal for permit applications, but not every application type is available online; if yours isn't on the portal, you schedule a submittal appointment to apply in person instead¹². The Building Division's own permit-type list also includes a floodplain development permit application, so a lot inside a mapped floodplain goes through the same office¹². Washington's Department of Ecology directs anyone doing development work in a floodplain to get a floodplain development permit from their local government, and some local governments regulate a larger area than FEMA's flood maps show, so confirm your parcel's flood status with Mason County before you plan a site¹³. The county's building-permit fee schedule sets a minimum permit fee of $156, and plan review is generally 65% of the permit fee¹⁴.

Yes. Mason County plans under Washington's Growth Management Act and administers a zoning code, Title 17, through the Community Development Planning Division at the same 615 W Alder St office^2,15. The county's zoning layers are Rural Zoning plus separate Allyn, Belfair and Shelton urban-growth-area zoning, and incorporated cities may run their own². In Rural Residential 5 (RR 5), the county allows one principal residence per 5 acres and one accessory dwelling unit per parcel, with a 2-acre minimum lot size unless the lot is in an approved performance subdivision¹⁶. Rural Residential 2.5 (RR 2.5) allows one dwelling per 2.5 acres, also with one accessory dwelling unit and a 2-acre minimum lot¹⁶. In both districts the front setback is 25 feet and the side and rear setback for a dwelling is 20 feet, or 50 feet for agricultural and home-occupation structures¹⁶. Ask Planning which zone your parcel sits in before you buy. Washington's state building code separately sets a minimum dwelling size wherever it applies: at least 190 square feet of habitable space, with at least one room of 120 square feet or more, and no sleeping or other habitable room smaller than 70 square feet, except kitchens, which have no minimum⁶. A tiny house on wheels can be built up to 400 square feet, including its kitchen, bathroom and sleeping area, and must be built to the state building code, but where you can place one, and whether it needs a foundation permit, depends on the local building department's review⁹.

Anyone installing, modifying or repairing an onsite sewage system in Mason County must first get a permit from Mason County Community Services, Environmental Health³. An ordinary Mason County septic permit is valid for three years from the site-inspection signature date, while a repair permit for a failed component is valid for one year from that date³. Mason County also requires a certified operations-and-maintenance specialist's inspection report dated within the preceding 12 months before a property sale, a residential or commercial building-permit application, a land division or a boundary-line adjustment¹⁷. Pit toilets are not permitted as sewage disposal in Mason County; another onsite system may be possible only with county and state approval¹⁷. Only about 1% of Mason County soils are rated free of severe limits for a conventional drain field, so arrange the site and soil evaluation before treating a parcel as buildable⁴.

This isn't confirmed for Mason County specifically, so treat it as an open question to ask Mason County Community Development Planning before you buy or move an RV or tiny home onto raw land¹⁵. Washington's manufactured-home community law defines a recreational vehicle as transient living quarters not occupied as a primary residence and not permanently affixed to a lot, but that definition governs mobile-home-park rules, not whether a county allows RV living on a private parcel, and it does not grant statewide permission to live in an RV on your own land¹⁸. A tiny house built to the state building code, up to 400 square feet, is a recognized dwelling type statewide, but where it can sit, and what foundation or utility connections it needs, is still a local building-department decision⁹. Mason County's own zoning code, administered by Planning, is what actually answers whether your specific parcel's zone allows RV or tiny-home occupancy and for how long².

1. Confirm your parcel's zoning district and any critical-area or floodplain overlay with Mason County Community Development Planning before you buy¹⁵.
2. Get a septic site and soil evaluation from Mason County Environmental Health early, since most of the county's soils are rated unsuitable for a conventional drain field and an engineered system takes more design time and money^4,19.
3. If your only road frontage is a county road, apply for a Public Works access permit before you build a new driveway approach or move an existing one^7,8.
4. If your frontage is a state highway instead, get written permission from the Washington State Department of Transportation before building the approach²⁰.
5. Apply for your building permit through the Building Division's SmartGov portal, or schedule an in-person submittal appointment for application types the portal doesn't cover¹².
6. For any construction costing more than $5,000, be ready to supply the parcel's legal description or tax parcel number, your contact information and your contractor's registration number as part of a complete application²¹.
7. If you're subdividing rather than building on an existing lot, know that every subdivision and short subdivision in Washington must meet chapter 58.17 RCW and Mason County's local subdivision regulations before the county will approve a plat²².

Soils and septic capacity come first: with only about 1% of the county's soils rated free of severe limits for a conventional drain field, get a site evaluation from Mason County Environmental Health before you assume a parcel can support a standard system^4,19. For a property with an existing septic system, ask for the certified inspection report that Mason County requires to be no more than 12 months old before a sale¹⁷. Check flood risk directly with Mason County, since local governments can regulate a larger area than FEMA's flood maps show and any development in a mapped floodplain needs its own permit¹³. Confirm road frontage and access: a driveway onto a county road needs a Public Works access permit, and one onto a state highway needs separate written WSDOT permission^7,20. Mason County is about 84.1% wooded and roughly 26.9% federal land⁴. The median property tax bill is about $2,833 a year, the effective property-tax rate is near 0.8%, and the typical annual homeowners insurance premium is roughly $1,242⁴.