ADUs in Unincorporated Spokane County

Identify your jurisdiction first

Spokane County contains several incorporated cities, including Spokane, Spokane Valley, Cheney, Liberty Lake, and Airway Heights. Each writes its own ADU rules. The guidance here applies only to unincorporated county land.

How unincorporated county ADU rules work

Unincorporated Spokane County regulates ADUs through its zoning code (Title 14) and the Building and Planning department. The rules depend on your parcel's zone and whether it sits inside the urban growth area.

Inside the urban growth area, Washington's HB 1337 requires the county to allow at least two ADUs per lot, with no owner-occupancy requirement and impact fees capped at 50 percent of the principal unit. Where the local code has not yet caught up to state law, state law controls.

Outside the urban growth area, the county sets its own standards for size, height, setbacks, and parking, and some zones may require a conditional use permit. A separate state law, HB 1345, lets counties choose whether to allow a detached ADU on rural parcels.

Because the exact requirements depend on your specific parcel, confirm the current standards with Spokane County Building and Planning before you design or apply, and check whether your lot can support an ADU.

Septic and water

Many county lots use a septic system and sometimes a private well rather than municipal sewer and water. Septic capacity can be a deciding feasibility factor for a county ADU and is reviewed by the Spokane Regional Health District, with water service confirmed through the serving purveyor. It is worth checking early.

The permit process in the county

ADU permits for unincorporated parcels run through Spokane County Building and Planning. To confirm the current rules for your specific lot, including its urban growth area status, use the county zoning code or contact the county Permit Center at 509.477.3675. Our homeowner permit guide explains what to expect.