Spokane County, WA ADU (accessory dwelling unit) rules and regulations (2026)

In Spokane County, the rural ADU standards are settled and specific. The urban ADU rules are still changing. The printed urban residential standards haven’t been rewritten yet for the state’s 2023 ADU law, and the county is working through that update now. Which set applies to your lot depends on where it sits and when you apply.

The rules below cover unincorporated Spokane County, the land outside city limits. For the statewide rules, see our Washington ADU laws guide.

City or county first

The City of Spokane, Spokane Valley, Liberty Lake, Cheney, Airway Heights, and the county’s other cities write their own ADU rules. If the parcel is inside one of them, the county zoning code doesn’t apply.

Unincorporated Spokane County includes urban-zoned land inside the urban growth area. It also includes rural and resource land. Building and Planning reviews both. The Spokane Regional Health District reviews septic.

The timing question

Washington’s ADU law applies inside urban growth areas. Under RCW 36.70A.680, each county’s ADU ordinance takes effect with its next periodic comprehensive plan update. Where a county hasn’t adopted the required rules, the state requirements “supersede, preempt, and invalidate” conflicting local rules. That means the state rules take over. RCW 36.70A.130 sets Spokane County’s update for on or before December 31, 2026.

In February 2026, county planners briefed the Planning Commission on what the 2046 Comprehensive Plan code work has to include:

• Two ADUs per qualifying urban lot, in any attached/detached mix.
• A size cap of at least 1,000 sq ft.
• The state’s 24-foot roof-height floor, meaning the height limit can’t be set lower than that.
• Alley siting.
• Parking limits.
• Impact fees capped at half of the main home’s.

That briefing describes changes the county has to make. It isn’t adopted county code.

What that means for you: on an urban-zoned unincorporated lot, ask Building and Planning which standards will govern your application before you design. It will be either the printed county code or the state requirements. The answer can change the number of units, the size, and whether you need a hearing.

Who reviews your lot

Rural zones: detached ADUs

In the Rural-5, Rural Traditional, Rural Activity Center, Urban Reserve, and Rural Conservation zones, SCC 14.618.230 allows a detached ADU with these standards:

• One ADU per lot with an existing single-family home. It can’t go on a lot with a duplex, an attached ADU, or an approved cluster development lot.
• Within 150 feet of the main house.
• Two bedrooms maximum and 800 sq ft on the main (ground) floor.
• Pitched roof with at least a 4:12 slope, and a ridge no higher than 24 feet.
• Title notice saying the ADU can’t be sold as a separate residence unless it becomes the only residence on a legally subdivided parcel.
• The ADU doesn’t count toward density, the number of homes allowed on the land.

The size limit is written for the main (ground) floor. Thinking about a loft or a second level? Ask Building and Planning how they’ll measure it before you design around it.

Rural and resource zones: attached ADUs

In the rural zones and the Large Tract Agricultural, Small Tract Agricultural, and Forest Lands zones, an attached ADU is allowed with standards:

• A complete, separate living unit attached to the house by a shared wall.
• One accessory unit within or attached to the house.
• No more than 35 percent of the building’s total livable floor area or 900 sq ft, whichever is less, with no more than two bedrooms.
• The house and ADU together need at least 1,200 sq ft of livable floor area.
• One off-street parking space for the ADU, on top of the house’s parking.
• The building still has to look like a single-family home. The second entrance goes on the side or rear, or somewhere it doesn’t stand out from the front.

The resource-lands use table lists attached ADUs but not detached ones.

Urban residential zones under the printed code

The 2025 printing of the county zoning code treats ADUs in the LDR, LDR-P, MDR, and HDR zones this way:

• Attached ADU: a limited use with the same core standards as above. That means one unit, 35 percent or 900 sq ft, two bedrooms, and a 1,200 sq ft combined minimum. It needs one off-street space and can’t go on a lot with a detached ADU, duplex, or multifamily dwelling.
• Detached ADU: a conditional use, which needs a Hearing Examiner decision. The code sets its own standards: two bedrooms and 800 sq ft on the main floor, a 4:12 roof, and a 24-foot ridge. It also requires one off-street space, no lot under 10,000 sq ft, and any conditions the Examiner sets.

Those urban rules clash in several places with the state ADU rules for urban growth areas. The state rules call for two units, a size cap of at least 1,000 sq ft, and limits on parking and design review (RCW 36.70A.681). That’s why the timing check above comes first.

Septic, water, and site

If the lot is on septic, the Health District is part of the project. The Spokane Regional Health District permits and inspects on-site sewage systems. It works with county building and planning staff.

A second home adds wastewater flow, so the septic answer usually decides the bedroom count and the location. Get that answer before you choose a plan.

Also check driveway access and slope, critical areas and surface water, and wildfire-area rules. Any one of them can limit where the home goes, even where zoning allows the ADU.

How a factory-built ADU fits

Wolf homes are modular homes built to Washington’s residential code. They aren’t manufactured homes built to the HUD code, the federal standard for manufactured homes.

Washington L&I reviews the factory plans and oversees factory inspections. Each home carries its L&I insignia before it ships. County review still covers zoning, the foundation, utility connections, and site inspections.

For rural detached ADUs, test any floor plan against two numbers first: the 800 sq ft main-floor limit and the 4:12 roof pitch. Compare our models with both.

Spokane County is far outside the standard 20-mile delivery in Wolf’s published installation scope. Transport is quoted per project after the site review. For pricing, see our cost page and the Washington ADU cost guide.

What to have ready

1. Address or parcel number, and whether it’s inside a city.
2. The zone: urban residential, rural, or resource land.
3. Septic permit and as-built drawing, or sewer information. Well records if you have a well.
4. Distance from the house to where you want the ADU, and the driveway route.

Send us the address for a free property evaluation. We’ll confirm which rule set and timing apply, and which models may fit, before you pay for design.