In plain words

Washington sets detailed statewide rules for ADUs. An ADU (accessory dwelling unit) is a second, smaller home on the same lot as a house. It can be inside the house, attached to it, or a separate backyard cottage. A 2023 law, HB 1337, covers most cities and counties. They must allow two ADUs on any lot where a house is allowed. This applies inside urban growth areas, the zones around cities where growth is planned. The two ADUs can be two cottages, two attached units, or one of each. Your town cannot require you to live on the property. It cannot cap an ADU below 1,000 square feet or 24 feet tall. Setback rules, meaning how far a building must sit from the property line, cannot be stricter than for the house. Impact fees, one-time charges for roads and parks, are capped at half of what a new house pays. Near frequent transit, towns cannot require a parking space. A 2025 law also stops larger cities from requiring parking for small homes, once they put it in place. A town cannot block selling an ADU separately as a condominium just because it was built as an ADU. Each city had to update its code by its planning deadline, from late 2024 to mid-2027. If it missed the deadline, the state rules apply anyway. Rural land is different. Since June 2026, counties may choose to allow one detached ADU up to 1,296 square feet on rural parcels. Towns can still limit short-term rentals. Lots with wetlands, steep slopes or no sewer may face extra limits. You still need permits, and your own lot and local code decide what fits.

The rules at a glance

State ADU lawYes
Where the state rule appliesCities, towns and counties that fully plan under the Growth Management Act (which includes all the big metro areas), but only on land inside urban growth areas. Rural land outside urban growth areas is covered only if the county opts in under the 2026 rural ADU law.
Approved without a hearing?Partly. Some ADUs or some places get staff approval; others can still need a hearing.
Kinds of ADU the state protectsinternal, attached, detached
ADUs per lot2 ADUs per lot inside urban growth areas (two attached, two detached, or one of each); 1 rural detached ADU per parcel where a county opts in
Owner living on siteTowns can’t require the owner to live on the property.
ParkingInside urban growth areas (ADU law): no off-street parking may be required within a half mile walk of a major transit stop; elsewhere no more than 1 space per unit on lots under 6,000 sq ft and no more than 2 per unit on larger lots. Exceptions exist near the largest commercial airport and where the state certifies a local safety study. Separately, a 2025 parking law (ESSB 5184) bars cities over 30,000 people and counties from requiring any parking for residences under 1,200 sq ft, which appears to cover most ADUs once each place implements it (by early 2027 for places of 50,000+, mid-2028 for places of 30,000-50,000).
SizeCities and counties may not cap an ADU below 1,000 sq ft of floor area, and may not limit ADU height below 24 ft (or below the house's limit if that is lower). Rural detached ADUs, where allowed, are capped at 1,296 sq ft.
Lot size and setbacksAn ADU must be allowed on any lot big enough for the main house. Setbacks, lot coverage, tree rules, door location and design review cannot be stricter than for the house. Detached ADUs may sit on an alley lot line unless the city plows that alley. Garages and other existing buildings can be converted even if they break current setback or coverage rules. Lots of 2,000 sq ft or less can be limited to two ADUs. The rules do not apply on lots with critical areas or their buffers, or in watersheds that feed a drinking-water reservoir listed as impaired, and localities may bar ADUs on lots without public sewer.
FeesImpact fees on an ADU cannot exceed 50% of the impact fees charged for the main house. Cities and counties cannot require public street improvements for an ADU.
Short-term rentalsCities and counties may restrict ADUs from being used as short-term rentals (under 30 nights).
Can towns opt out?No

What to do next

  1. Check whether your lot is inside an urban growth area; your city or county zoning map or permit counter can confirm.
  2. Find your city's or county's updated ADU code and confirm it took effect (deadlines ran from December 31, 2024 to June 30, 2027 depending on county).
  3. Look for pre-approved ADU plans or an architect self-certification program, which can shorten permit review (Seattle offers pre-approved backyard cottage plans).
  4. Ask for an impact fee estimate and confirm it is no more than half of the fee for a new house.
  5. If you live in King County or a similar-size county and plan to rent to a lower-income tenant, ask the county about the ADU property tax exemption before you build.
  6. On rural land, ask whether your county has opted in to the 2026 rural detached ADU rules; if not, the older county rules apply.

Grants, loans and tax breaks

  • Low-income ADU property tax exemption (RCW 84.36.400(2)): King County, and counties of 900,000 to 1.5 million people (and their cities) that pass a resolution, may exempt an ADU's value from property tax while it is rented to a household earning 60% of area median income or less, at rent no higher than 30% of the tenant's income, on a lease of at least 12 months. In the mid-size counties only detached ADUs qualify. Immediate family members do not qualify as tenants. Program page
  • Home improvement property tax exemption (RCW 84.36.400(1)): Statewide, an improvement to a single-family home, including building an ADU, can be exempt from property tax for three years if it adds no more than 30% of the original structure's value. You must file notice with the county assessor before you build, and you can claim it once every five years. Program page

What your town still decides

Each city and county had to update its code by its periodic comprehensive plan deadline: December 31, 2024 for King, Kitsap, Pierce and Snohomish; December 31, 2025 for the Clark, Thurston, Whatcom group; December 31, 2026 for the Spokane, Yakima, Benton group; and June 30, 2027 for the remaining counties. Where a city misses its deadline, the state rules override conflicting local rules automatically. Within the state floor, cities still set details such as design standards that also apply to houses, short-term rental limits and permit fees. Small and rural counties that only partly plan under the Growth Management Act are not covered.

Examples of local rules

  • Seattle: Allows up to two ADUs on a property, even on lots big enough for more homes. No parking or street improvements are required for ADUs. The city's ADUniverse site offers pre-approved backyard cottage plans that can be permitted in about 2 to 6 weeks in most cases. Source
  • Spokane: City code caps detached ADUs at 1,000 sq ft. If a lot has both an ADU and a short-term rental, an owner must live on site as a primary residence. Updated by Ordinance C36696, effective July 11, 2025. (The city code site blocks automated readers; these points were confirmed from the indexed text of the same code section.) Source

What we’re still confirming

  • Some cities can avoid the state's ADU parking limits with a state-approved parking safety study; ask yours whether it has one.
  • Whether every city applies the 2025 ban on parking minimums for homes under 1,200 sq ft to ADUs isn't confirmed; ask your city.

Laws, bills and citations

The laws behind this (8)
  • Detached ADUs outside urban growth areas (county option) (EHB 1345 (2026), Laws of 2026 ch. 231; new RCW 36.70A.685, amends RCW 36.70A.696, signed March 27, 2026, in effect June 11, 2026). Lets counties that fully plan under the Growth Management Act allow detached ADUs on rural parcels if they choose. Each parcel may have only one ADU of any kind. A detached ADU is capped at 1,296 sq ft, must sit within 150 ft of the house, share its driveway, have metered water and adequate septic, and cannot go on nonconforming lots under one acre. Counties that opt in must also run code enforcement, track permits and cap how much rural growth ADUs can absorb. County ADU rules adopted before the law stay valid. Read it
  • Statewide limits on minimum parking requirements (ESSB 5184 (2025), Laws of 2025 ch. 204 (new sections in RCW 35.21, 35A.21, 36.01; repeals RCW 36.70A.620), signed May 7, 2025, in effect July 27, 2025). Not ADU-specific. Bars cities over 30,000 people and counties from requiring any off-street parking for residences under 1,200 sq ft and from requiring more than one space per single-family home, with exceptions near the largest airport and where the state certifies a local safety study. Read it
  • ADU property tax exemption expansion (ESB 5529 (2025), Laws of 2025 ch. 207; RCW 84.36.400, signed May 7, 2025, in effect July 27, 2025). Lets counties of 900,000 to 1.5 million people, and cities in them, join King County in offering a property tax exemption for ADUs rented to low-income households. The local government must pass a resolution to offer it, and in those mid-size counties it covers detached ADUs only. Read it
  • Condominium construction simplification (EHB 1403 (2025), Laws of 2025 ch. 201, signed May 7, 2025, in effect July 27, 2025). Exempts ADUs set up as condominium units from the building enclosure design and inspection requirements that apply to other new condominiums, which makes selling an ADU separately less costly. Read it
  • Growth management update deadlines (SSB 5558 (2025), Laws of 2025 ch. 148 (amends RCW 36.70A.680 and 36.70A.130), signed April 22, 2025, in effect July 27, 2025). Makes the 2023 ADU rules take effect at the same time as each city's or county's next periodic comprehensive plan update, instead of six months later. Moved the deadline for jurisdictions with 2026 updates to December 31, 2026. Read it
  • ADU permit self-certification (SHB 1353 (2025), Laws of 2025 ch. 22, signed April 7, 2025, in effect July 27, 2025). Lets cities run optional programs where a registered architect certifies that a detached ADU meets building code, which can speed up permits. Cities that run one must report results to the state Commerce department. Read it
  • Expanding housing options through ADUs (HB 1337 (2023), Laws of 2023 ch. 334; RCW 36.70A.680, 36.70A.681, 36.70A.696, signed May 8, 2023, in effect July 23, 2023). Requires cities and counties that fully plan under the Growth Management Act to allow two ADUs on every lot that allows a house inside urban growth areas. It bans owner-occupancy rules, caps impact fees at half of what a house pays, sets minimum size and height allowances, limits parking rules, and allows ADUs to be sold as condominiums. Read it
  • ADU parking near transit (ESSB 6617 (2020), Laws of 2020 ch. 217; RCW 36.70A.696-.698, signed March 27, 2020, in effect June 11, 2020). Stopped cities in growth-planning counties from requiring off-street parking for ADUs within a quarter mile of a major transit stop, with limited exceptions. HB 1337 later widened this to a half mile for cities and counties it covers. Read it
Proposed and failed bills (2). None of these is law.
  • SB 5413 (2025-26): died as of October 3, 2026. Senate version of the rural detached ADU bill. It never left the Senate Housing Committee and died when the 2026 session ended; the House version (EHB 1345) became law instead. Bill page
  • SB 5235 (2023-24): died as of October 3, 2026. Senate ADU reform bill. It passed the Senate in 2023 but never got a House floor vote; it was returned to the Senate Rules 'X' file in January 2024 and died. The House bill HB 1337 became law instead. Bill page

Sources

  1. EHB 1345 (2026), Laws of 2026 ch. 231; new RCW 36.70A.685, amends RCW 36.70A.696 · official source
  2. ESSB 5184 (2025), Laws of 2025 ch. 204 (new sections in RCW 35.21, 35A.21, 36.01; repeals RCW 36.70A.620) · official source
  3. ESB 5529 (2025), Laws of 2025 ch. 207; RCW 84.36.400 · official source
  4. EHB 1403 (2025), Laws of 2025 ch. 201 · official source
  5. SSB 5558 (2025), Laws of 2025 ch. 148 (amends RCW 36.70A.680 and 36.70A.130) · official source
  6. SHB 1353 (2025), Laws of 2025 ch. 22 · official source
  7. HB 1337 (2023), Laws of 2023 ch. 334; RCW 36.70A.680, 36.70A.681, 36.70A.696 · official source
  8. ESSB 6617 (2020), Laws of 2020 ch. 217; RCW 36.70A.696-.698 · official source
  9. RCW 36.70A.680 Accessory dwelling units, local regulation · official source · checked 2026-10-03
  10. RCW 36.70A.681 Accessory dwelling units, limitations on local regulation · official source · checked 2026-10-03
  11. RCW 36.70A.685 Accessory dwelling units outside of urban growth areas · official source · checked 2026-10-03
  12. RCW 36.70A.696 Accessory dwelling units, definitions · official source · checked 2026-10-03
  13. RCW 36.70A.697 and 36.70A.698 ADU parking near transit · official source · checked 2026-10-03
  14. RCW 36.70A.130 comprehensive plan update deadlines · official source · checked 2026-10-03
  15. RCW 84.36.400 property tax exemption for improvements and ADUs · official source · checked 2026-10-03
  16. SSB 5558 (2025) session law, section 4 · official source · checked 2026-10-03
  17. Summary of Legislation, 2025 Session (Office of Program Research) · official source · checked 2026-10-03
  18. Summary of Legislation, 2026 Session (Office of Program Research) · official source · checked 2026-10-03
  19. Washington Department of Commerce ADU guidance page · official source · checked 2026-10-03
  20. Seattle SDCI accessory dwelling units · official source · checked 2026-10-03
  21. Seattle ADU code updates (HB 1337 compliance) · official source · checked 2026-10-03
  22. Spokane Municipal Code 17C.300.110 · official source · checked 2026-10-03
  23. EHB 1345 (2026) session law, Laws of 2026 ch. 231 · official source · checked 2026-10-03
  24. Governor's partial veto message, HB 1337 (2023) · official source · checked 2026-10-03

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.