Guides / ADUs · Updated
What Washington lets you build as an ADU
From the Revised Code of Washington, RCW 36.70A.680 and 36.70A.681 (HB 1337, 2023), checked November 2026.
Short answer
Inside a Washington urban growth area, your city or county has to let you build two ADUs on a single-family lot, attached or detached. It can't cap them under 1,000 sq ft or 24 ft tall, can't make you live on the lot, and can't charge more than half the house's impact fees. Within a half-mile walk of frequent transit, no parking can be required.
Where it applies
The rules cover cities and counties that plan fully under the Growth Management Act, and "must only apply in the portions of towns, cities, and counties that are within urban growth areas" (RCW 36.70A.680(2)). They don't apply to lots in critical areas or their buffers, or in a watershed for an impaired drinking-water reservoir (36.70A.681(4)). A city may still limit ADUs on lots without public sewer and near septic systems or critical areas (36.70A.680(4)–(5)).
Each place's own rules had to match the law "at the same time as the jurisdiction's next periodic comprehensive plan update", and where a place hasn't acted, the state rules "supersede, preempt, and invalidate any conflicting local development regulations" (36.70A.680(1)). Those updates were due by December 31, 2024 for King, Kitsap, Pierce and Snohomish counties and their cities; December 31, 2025 for Clallam, Clark, Island, Jefferson, Lewis, Mason, San Juan, Skagit, Thurston and Whatcom; December 31, 2026 for Benton, Chelan, Cowlitz, Douglas, Franklin, Kittitas, Skamania, Spokane, Walla Walla and Yakima; and June 30, 2027 for the rest (36.70A.130(5)).
The rules, one by one
| Rule | What the law says | RCW |
|---|---|---|
| How many on a lot | At least two ADUs on every lot in a zone that allows single-family homes: one attached and one detached, two attached, or two detached (in one or two buildings) | 36.70A.681(1)(c) |
| Small lots | A city may cap a lot of 2,000 sq ft or less at two ADUs plus the house | 36.70A.681(3) |
| Size | No maximum floor area under 1,000 sq ft (interior living space, basements and attics included, garages not) | 36.70A.681(1)(f) |
| Height | No roof height limit under 24 ft, unless the house itself has a lower limit | 36.70A.681(1)(g) |
| Setbacks and design | Nothing stricter than for the house: setbacks, lot coverage, tree retention, door placement, looks or design review. Detached ADUs may sit on a lot line along a public alley | 36.70A.681(1)(h)–(i) |
| Converting a garage | Allowed from existing structures such as detached garages, even if they break today’s setback or coverage rules | 36.70A.681(1)(j) |
| Owner-occupancy | Can’t be required, for the ADU or any other unit on the lot | 36.70A.681(1)(b) |
| Parking | None within a half-mile walk of a major transit stop; otherwise at most 1 space per unit on lots under 6,000 sq ft and 2 on larger lots | 36.70A.681(2)(a) |
| Impact fees | No more than 50% of what the house would pay | 36.70A.681(1)(a) |
| Selling it | A city can’t stop an ADU being sold as a condo just because it was built as an ADU | 36.70A.681(1)(k) |
| Street improvements | Can’t be required to permit an ADU | 36.70A.681(1)(l) |
| Short-term rentals | Cities and counties may restrict them | 36.70A.680(5)(a) |
These are the floor. A city's code can be more generous, so read your city's ADU page too. A "major transit stop" includes a bus with service at least every 15 minutes for 5 peak hours on weekdays (RCW 36.70A.696).
In the law's words
A city or county "must allow at least two accessory dwelling units on all lots that are located in all zoning districts within an urban growth area that allow for single-family homes" (RCW 36.70A.681(1)(c)).
It "may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot" (36.70A.681(1)(b)).
It "may not establish a maximum gross floor area requirement for accessory dwelling units that is less than 1,000 square feet" (36.70A.681(1)(f)).
See it on your lot
Place an ADU on your lot with the measuring map, and price it with the ADU cost calculator. The ADU floor plans guide has six ready-made plans from 600 to 1,200 sq ft; any up to 1,000 sq ft fits under the smallest cap a Washington city may set. Other states: California, Oregon. Texas has no statewide ADU law; a 2025 bill (SB 673) passed the Senate but not the House, so each city sets its own rules.