Jurisdiction
ADUs in Unincorporated Spokane County
These rules apply to parcels in unincorporated Spokane County, meaning lots not inside any city. The key questions are your parcel's zone and whether it sits inside an urban growth area, because HB 1337 applies inside the urban growth area but the county sets its own standards outside it.
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 (Chapter 231, Laws of 2026, effective June 11, 2026), lets a county choose whether to allow a detached ADU on parcels outside the urban growth area. It is an opt-in for the county, not a right for the owner, and where a county does opt in the law caps the parcel at one ADU total, caps the detached unit at 1,296 square feet of gross floor area, requires it within 150 feet of the principal unit on the same driveway, requires metered water and documentation that the septic system can take the extra demand, and bars it on nonconforming lots under one acre. Ask Spokane County Building and Planning whether the county has adopted such an ordinance.
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.
Unincorporated Spokane County ADU questions
- How do I know if my lot is in unincorporated Spokane County?
- If your parcel is not inside any city, the unincorporated Spokane County rules apply. Cities within the county, including Spokane, Spokane Valley, Cheney, Liberty Lake, and Airway Heights, have their own codes. Identify your jurisdiction first, because the rules differ.
- Does HB 1337 apply in unincorporated Spokane County?
- It applies to the urban growth area (UGA) portions of the county, where two ADUs per lot and no owner-occupancy requirement apply under HB 1337. Outside the UGA, the county sets its own standards. So the first question for a county lot is whether it is inside the UGA.
- What are the Spokane County BP-75 and BP-76 sheets?
- BP-75 (detached) and BP-76 (attached) are Spokane County Building and Planning information sheets for ADUs. They are a helpful starting point, but the controlling source is the county zoning code (Title 14) and the Building and Planning department, not the brochure.
References and primary sources
Regulatory and fee specifics on this page are cited to the current primary sources below. Confirm any figure against its source for your specific parcel before you rely on it.
- Spokane County Codes and Ordinances, Zoning Code Title 14 (spokanecounty.gov) The controlling ADU source for unincorporated county land.
- Spokane County BP-75, Detached ADU (spokanecounty.gov) Orientation sheet, not the controlling source.
- Spokane County BP-76, Attached ADU (spokanecounty.gov) Orientation sheet, not the controlling source.
- RCW 36.70A.680 and .681 (app.leg.wa.gov) HB 1337 and its urban growth area applicability.
- Engrossed House Bill 1345, Chapter 231, Laws of 2026 (lawfilesext.leg.wa.gov) Detached ADUs outside urban growth areas, county opt-in, effective June 11, 2026.
Related
What it costs, and what happens after you send it
- What does an ADU cost in Spokane County?
- detached new-build $110,000 to $260,000, garage conversion $50,000 to $120,000, prefab or modular installed $80,000 to $180,000. Add roughly $3,000 to $5,000 in permit, plan review and impact fees, which is an estimate from one source rather than a published fee schedule, so confirm it with your permitting jurisdiction.
- How solid are those numbers?
- Treat them as planning ranges, not prices. They come from two third-party estimator sites, ADU Zoning and ADU Wizard, both read in full on 2026-08-17, neither of them a government publication. The two land in nearly the same place on total project cost, which is why totals are shown here. They differ by roughly a factor of two on cost per square foot, so a per-square-foot rate is not a reliable way to budget an ADU here and none is quoted.
- How fast will someone get back to me?
- Your request goes to an independent local ADU builder, usually within about an hour during the day. The builder then contacts you to look at the property and price the project on their own schedule.
- What types are covered?
- Detached ADUs, attached ADUs, garage conversions and basement junior ADUs. See every type.
Only a builder standing on your lot can price your actual project.
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