ADU (accessory dwelling unit) Rules and Regulations in Mason County, WA (2026)

On rural land in Mason County, an ADU (accessory dwelling unit) needs a special use permit, according to the county’s ADU application materials. The county reviews each application against health, safety, and site criteria before approval, so it pays to know the process before you commit to a model or a budget.

The rules below are the rural standards in Mason County Code 17.03.029. A Shelton mailing address does not establish jurisdiction: property inside the city or an urban growth area needs a separate review. Washington’s urban ADU provisions should not be assumed to apply to rural land.

Where ADUs are allowed

The special use permit is the standard rural route. If your property is in shoreline jurisdiction, the county’s materials direct applicants to a shoreline review path instead of the standard rural special use route.

A few baseline limits apply:

• You are allowed one ADU per property.
• The ADU must be located on the same lot as the primary residence.
• Recreational vehicles are not allowed as ADUs under this process.

Size limits

Mason County ties ADU size to the size of the primary residence, with a hard cap on top of that.

• For a new ADU, the unit can be no greater than 80% of the primary residence or 1,200 square feet, whichever is smaller.
• If you convert an existing residence into the ADU while developing a new primary residence, the converted unit can be no greater than 80% of the primary residence or 1,500 square feet, whichever is smaller.

The “whichever is smaller” language matters: depending on the size of your primary residence, either the 80% figure or the square-footage cap can be the binding limit.

Setbacks

The ADU must meet applicable setback requirements. Separately, the county looks at separation between the foundation and septic components, including drainfields, reserve areas, and septic tanks. Confirm the exact distances for your parcel.

Septic, well, sewer, and water

This is the part of a Mason County ADU that tends to control the timeline, because Environmental Health approval is required before the ADU permit can be approved.

If you are using an existing onsite sewage system (OSS):

• The total bedroom count across the existing and proposed connected structures must match the approved septic records on file.
• If the bedroom count exceeds the approved system size, the county directs applicants to work with a licensed septic designer on upgrade options.

If you need new infrastructure:

• If a new septic system is needed, the OSS application and design permit must be submitted and approved before Environmental Health will approve the ADU.
• If a new well is needed, it must be permitted and constructed before Environmental Health approval of the ADU permit.

For sewer or larger public water systems, signed adequacy forms are required from the system manager.

Owner-occupancy and rental

The current rural code requires the owner to live in the principal residence or ADU when the property is in shoreline jurisdiction. It does not impose that requirement on every rural ADU. The older application packet uses broader wording; use the current code and confirm shoreline status with Planning. Rental use and any recorded restrictions still need a separate check.

Parking

The county requires one additional off-street parking space for the ADU.

Permit process and what gets reviewed

Expect a site plan to be central to the application. The site plan needs to show property lines, easements, rights-of-way, existing and proposed structures, setbacks, access, parking, septic components, wells, waterlines, and stormwater routes.

The county evaluates the application against review criteria tied to health and safety, compatibility with the Comprehensive Plan, hazardous conditions, adequacy of public facilities, impacts on adjacent properties, and whether the project would require extension of urban services outside an Urban Growth Area. As part of that evaluation it also reviews whether the site is in a floodplain and whether critical site features such as wetlands, streams, steep bluffs, and bodies of water are present on the property.

Mason County’s linked ADU packet is an older checklist, useful for identifying water, sewage, and site-plan records. Confirm its current submission steps and fees with the county; where its zoning language differs, use the current code.

What Wolf checks first

For a Mason County project, we usually want to know early:

1. Does the rural special use permit path apply, or is the property in shoreline jurisdiction?
2. How large can the ADU be after applying both the 80%-of-primary rule and the square-footage cap?
3. What does the approved septic record say, and does the bedroom count leave room for the ADU?
4. Is there an existing well and OSS, or will new infrastructure need to be permitted first?
5. Can a modular unit be delivered, staged, and set safely on the site?

We answer those before anyone talks about finishes. A unit that looks right on paper can still stall on septic capacity or an Environmental Health hold.

Get a free Wolf property evaluation: send us the address and how you plan to use the unit, and we’ll review jurisdiction, size limits, septic and well capacity, and model fit.

This is general information, not legal advice. Confirm current rules, fees, and application requirements with Mason County before you build.