Washington's ADU rules, codified at RCW 36.70A.680 and .681, changed what most residential lots inside urban growth areas can hold. For owners and builders, the question has moved from whether an ADU is allowed to how many, and on what terms.
What the state rules set
Two ADUs per lot. Attached, detached, or one of each, or converted from existing structures.
Size. Jurisdictions must allow an ADU of at least 1,000 square feet.
Owner occupancy. It cannot be required, except where the ADU is a short term rental.
Impact fees. Capped at 50 percent of the impact fees charged for the principal unit.
Sale. ADUs may be sold as independent units through a condominium or unit lot subdivision.
Parking. Requirements can be limited based on distance from a major transit stop and lot size.
What still varies by city
The state rules set floors, not a uniform code. Setbacks, height, lot coverage, design standards and critical areas rules still come from the local code. Many cities have rewritten their ADU chapters, and some went beyond the state minimums.
Rural lots
Outside urban growth areas, rules differ. Many counties may allow one rural detached ADU per lot, with restrictions. Check the county code before assuming the urban rules apply.
Before you design
Confirm the lot is inside an urban growth area and which jurisdiction issues the permit.
Check utilities. Water, sewer and electrical capacity can decide the second ADU before zoning does.
Check critical areas and easements, which shrink the buildable area.
Decide whether the units will ever be sold separately. It changes the land division path and the utility layout.
Frequently Asked Questions
Can I build two ADUs on my lot in Washington?
Inside urban growth areas, most cities and counties must allow two ADUs per residential lot, attached, detached or converted.
Do I have to live on the property to build an ADU?
Owner occupancy cannot be required, except where the ADU is a short term rental.
Can ADUs be sold separately?
Yes. The state rules allow ADUs to be sold as independent units through a condominium or unit lot subdivision.
Sources
Insights are general information researched from published sources on the date shown. Rules change, so confirm with the permitting authority before you act.
