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

In Spokane County, the urban ADU rules are still in transition. The rural ADU standards are settled and specific. The printed urban residential standards haven’t yet been rewritten 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.

This summary covers unincorporated Spokane County. 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 govern.

Unincorporated Spokane County includes urban-zoned land inside the urban growth area as well as 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, and where a county hasn’t adopted the required rules, the state requirements “supersede, preempt, and invalidate” conflicting local regulations. 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 must include: two ADUs per qualifying urban lot in any attached/detached mix, a size cap of at least 1,000 square feet, the state’s 24-foot roof-height floor, alley siting, parking limits, and impact fees capped at half of the principal unit’s. That briefing describes required changes, not adopted county code.

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

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. Not on a lot with a duplex, an attached ADU, or an approved cluster development lot.
• Within 150 feet of the primary residence.
• Two bedrooms maximum and 800 square feet on the main (ground) floor.
• Pitched roof with at least a 4:12 slope, and a ridge no higher than 24 feet.
• Title notice stating the ADU can’t be sold as a separate residence unless it becomes the sole residence on a legally subdivided parcel.
• The ADU doesn’t count toward density.

The size limit is written for the main (ground) floor. If you are considering a loft or a second level, ask Building and Planning how it will be measured 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 housekeeping unit attached to the house by a common 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 square feet, whichever is less, with no more than two bedrooms.
• The house and ADU together must have at least 1,200 square feet of livable floor area.
• One off-street parking space for the ADU, in addition to the house’s parking.
• The building must still read as a single-family home, with the second entrance on the side or rear or otherwise unobtrusive 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: one unit, 35 percent or 900 square feet, two bedrooms, a 1,200-square-foot combined minimum, one off-street space, and not on a lot with a detached ADU, duplex, or multifamily dwelling.
• Detached ADU — a conditional use. The code requires a Hearing Examiner decision and sets its own standards: two bedrooms and 800 square feet on the main floor, a 4:12 roof and 24-foot ridge, one off-street space, no lot under 10,000 square feet, and any conditions the Examiner imposes.

Those urban rules conflict in several places with the state ADU rules for urban growth areas: two units, a 1,000-square-foot minimum cap, and limits on parking and design review (RCW 36.70A.681). That is why the timing check above comes first.

Septic, water, and site

If the lot is on a septic system, the Health District is part of the project. The Spokane Regional Health District permits and inspects on-site sewage systems and works with county building and planning staff. A second dwelling 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 requirements. Any one of them can limit placement even where zoning allows the ADU.

How a factory-built ADU fits

Wolf homes are modular, built to Washington’s residential code — not HUD (the federal standard for manufactured homes) manufactured homes. Washington L&I reviews the factory plans and oversees factory inspections, and each home carries its L&I insignia before shipping. County review still covers zoning, the foundation, utility connections, and site inspections.

For rural detached ADUs, the 800-square-foot main-floor limit and the 4:12 roof pitch are the two numbers to test first against any floor plan. Compare our models with both. Spokane County is far outside the standard 20-mile delivery in Wolf’s published installation scope, so transport is quoted per project after the site review. See cost and the Washington ADU cost guide.

What to have ready

1. Address or parcel number, and whether it is inside a city.
2. The zone — urban residential, rural, or resource land.
3. Septic permit and as-built, 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 and we’ll confirm the rule set, the timing question, and the model fit before you pay for design.

This is general information, not legal advice. Recheck the current statute, local code, and permit packet before you file — jurisdictions amend forms and effective dates.