State ADU law
Washington ADU laws: two ADUs on every urban lot
Washington’s 2023 law (HB 1337) is one of the strongest in the country. It applies inside urban growth areas.
What to do next: use this research to check your own lot and bids, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsWritten by HyreADU Research Desk Primary-source statute research
The short answer
Washington ADU laws require cities and counties to allow at least two accessory dwelling units on every lot zoned for single-family homes inside an urban growth area. RCW 36.70A.681 lets owners build them attached, detached, or one of each.
A city may not require the owner to live on the lot. It may not cap ADU size below 1,000 square feet or height below 24 feet. Impact fees on an ADU cannot exceed half of what the main house would pay.
No off-street parking can be required within half a mile walking distance of a major transit stop. Cities had to adopt these rules with their next periodic comprehensive plan update. Where they have not, the state rules override conflicting local code (RCW 36.70A.680(1)(b)).
What Washington law requires of cities
Each row is from RCW 36.70A.681 (2023 c 334 s 4) as published by the Washington State Legislature.
| Question | What the Washington statute says | Citation |
|---|---|---|
| How many ADUs? | At least two per lot in zones allowing single-family homes in an urban growth area: one attached and one detached, two attached, or two detached. | RCW 36.70A.681(1)(c) |
| Owner-occupancy? | Banned. A city may not require the owner to live in the ADU or another unit on the lot. | RCW 36.70A.681(1)(b) |
| Size? | A city may not set a maximum gross floor area below 1,000 square feet. | RCW 36.70A.681(1)(f) |
| Height? | No roof height limit under 24 feet, unless the main house limit is lower. | RCW 36.70A.681(1)(g) |
| Setbacks and design? | No setback, lot coverage, tree retention, door location, design review or aesthetic rule stricter than for the main house. Garage conversions are allowed even if they break current setback or coverage rules. Detached ADUs may sit at an alley lot line unless the city plows that alley. | RCW 36.70A.681(1)(h)–(j) |
| Parking? | None within half a mile walking distance of a major transit stop. Elsewhere, at most one space per unit on lots under 6,000 sq ft and two on larger lots. Two narrow exceptions exist. | RCW 36.70A.681(2) |
| Impact fees? | No more than 50% of the impact fees for the main house. | RCW 36.70A.681(1)(a) |
| Sale as a condo? | A city may not bar selling an ADU as a condominium unit just because it began as an ADU. | RCW 36.70A.681(1)(k) |
Washington ADU rules set by state law, as read on the official statute page on October 27, 2026.
Who the law covers and when it took effect
The rules apply to cities and counties planning under the Growth Management Act, and only inside designated urban growth areas (RCW 36.70A.680(2)). Lots with critical areas or their buffers, and certain impaired drinking-water watersheds, are excluded (RCW 36.70A.681(4)).
Cities had to build the rules into their code to take effect with their next periodic comprehensive plan update under RCW 36.70A.130. Where a city has not done so, the statute supersedes and invalidates conflicting local rules (RCW 36.70A.680(1)(b)). The section history shows amendments in 2023 c 334 and 2025 c 148.
What is still up to your city or county
Cities may still restrict ADUs used as short-term rentals, apply health, building and environmental rules that also apply to the main house, and bar ADUs on lots not served by public sewer (RCW 36.70A.680(5)).
Outside urban growth areas, rural county rules govern and the two-ADU rule does not apply. For permit counts in Seattle, the state’s largest ADU market, see our Washington ADU permit data.
How to check your own city
Answer these in order, in writing, before you pay for drawings.
- 1 Confirm your lot is inside an urban growth area
Your county’s GIS map shows the UGA line. Outside it, the statute does not apply.
- 2 Check for critical areas
Wetlands, steep slopes and their buffers fall outside the rule. Ask for the critical-areas map for your parcel.
- 3 Ask whether the city’s code has been updated
If not, cite RCW 36.70A.680(1)(b): the state rules control where they conflict.
- 4 Measure distance to a major transit stop
Within half a mile walking distance, no parking can be required.
- 5 Ask about sewer
A city may bar ADUs on lots without public sewer. Confirm your connection first.
What this page does not cover
This page covers the Washington statute only. It does not cover building codes, septic and well rules, HOA covenants, flood or wildfire overlays, or your city’s fee schedule. Those can still stop a project the statute allows.
We read the statute on October 27, 2026. Laws change, and courts read them.
If your project depends on one clause, have a land-use attorney or the planning department confirm it for your parcel.
For the national picture, see which states have ADU laws; for what to ask at the counter, see the ADU permit guide.
Questions
How many ADUs can I build in Washington?
Does Washington require owner-occupancy for ADUs?
What is the size limit for an ADU in Washington?
Do I need parking for an ADU in Washington?
Written and audited by
HyreADU Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a builder. We read the permit extract, the statute, the HCD return or the fee schedule ourselves, and publish each figure with its source and retrieval date.
Where a number cannot be traced to a primary source, we leave it out and say what we could not verify. Our store-based claims cover California only.
- CA
- the only state this desk will make store-based claims about
- 5
- jurisdictions with extracted ADU permit evidence
- 735
- CSLB-verified companies in the California store
- 0
- national claims from a one-state store
How this desk works
- Primary sources only. Permit counts come from the city or county that issued the permit. Production counts come from HCD’s Annual Progress Report. Rents come from HUD or the Census. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- This is a California site. The company store is 734 California firms and one New Mexico firm. Permit evidence exists for five named jurisdictions: Los Angeles, San Francisco, Sacramento, San José and unincorporated Marin. A number from that store is titled to those places, never to the United States.
- A permit is not a completion, and a license is not an ADU grade. California licenses no ADU classification. Being named on an ADU permit is evidence of engagement in that jurisdiction, not of quality, completion, or work anywhere else. Owner-builder permits are excluded from contractor counts.
- Calculation is labeled as calculation. Figures we derive are never presented as something HCD, HUD, the Census or a city published. Terner Center research is cited as Terner’s, never restated as ours.
- We do not design, permit or build ADUs, and we take no payment for placement, ranking or a favorable mention. Pages that look like rankings are not: they publish public-record counts and let the reader decide.
- Nothing here is legal, tax or financial advice. Zoning, underwriting and appraisal practice vary by jurisdiction, lender and appraiser. The useful next step on a specific lot is the planning counter and a licensed professional.
Data as of RCW 36.70A.680–.681 read on app.leg.wa.gov, retrieved 2026-10-27.. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
- RCW 36.70A.681, Accessory dwelling units, limitations on local regulation , Washington State Legislature Retrieved October 27, 2026.
- RCW 36.70A.680, Accessory dwelling units , Washington State Legislature; compliance timing and local powers Retrieved October 27, 2026.
Planning an ADU in Washington?
Tell us about your lot and what you want to build. Matching is not live yet, so this is not a quote. We will send ADU options and the next questions to ask your city.
HyreADU is a publisher. It does not design, permit or build ADUs, and matching with builders is not live yet. This page describes state statutory text as published on the retrieval date.
It is not legal advice. Your city or county applies the rules to your lot, and its planning department is the authority for your parcel.