Washington's HB 1337, Chapter 334, Laws of 2023, took effect July 23, 2023. Inside urban growth areas, cities and counties must allow at least two ADUs on single-family lots, may not require owner occupancy, must permit at least 1,000 square feet, and may not charge ADU impact fees above 50 percent of the principal unit.
Last verified: October 18, 2026 against the Washington State Legislature RCW text (RCW 36.70A.680, 36.70A.681, 36.70A.130 and 36.70A.635), the enrolled HB 1337 and HB 1110 session laws, and the Washington Department of Commerce HB 1337 ADU guidance notice.
An accessory dwelling unit, or ADU, is a second, smaller home on a lot that already has a house, either attached to it or detached in the yard. A detached ADU in Washington is often called a DADU, or backyard cottage. This page explains what Washington’s statewide ADU law, HB 1337, allows, and how the companion middle housing law, HB 1110, connects to it.
What does Washington ADU law allow?
Washington’s ADU law is Engrossed House Bill 1337, Chapter 334, Laws of 2023. It added new sections to chapter 36.70A RCW, the Growth Management Act, and it took effect on July 23, 2023, per the effective date stated in the enrolled session law. Section 3 of the act is codified as RCW 36.70A.680, and section 4 as RCW 36.70A.681.
Inside an urban growth area, a city or county must allow at least two ADUs on all lots in zoning districts that allow single-family homes. RCW 36.70A.681(1)(c) requires a city or county to “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.” The allowed configurations are one attached plus one detached, two attached, or two detached. An urban growth area is the area a county designates for urban development under the Growth Management Act, and RCW 36.70A.680(2) confirms these requirements “must only apply in the portions of towns, cities, and counties that are within urban growth areas designated under this chapter.”
| Rule | Washington standard | Statute | Source |
|---|---|---|---|
| Number of ADUs | At least two ADUs per single-family lot in an urban growth area | RCW 36.70A.681(1)(c) | RCW 36.70A.681 |
| Owner occupancy | May not be required | RCW 36.70A.681(1)(b) | RCW 36.70A.681 |
| Maximum size floor | May not cap ADU size below 1,000 square feet | RCW 36.70A.681(1)(f) | RCW 36.70A.681 |
| Impact fees | May not exceed 50 percent of the principal unit’s impact fees | RCW 36.70A.681(1)(a) | RCW 36.70A.681 |
| Parking near transit | None required within one-half mile of a major transit stop | RCW 36.70A.681(2)(a)(i) | RCW 36.70A.681 |
| Roof height | May not limit ADU roof height below 24 feet | RCW 36.70A.681(1)(g) | RCW 36.70A.681 |
| Conversions | Existing structures, including detached garages, may convert even if they violate current setback or lot coverage code | RCW 36.70A.681(1)(d), (1)(j) | RCW 36.70A.681 |
| Where it applies | Only inside designated urban growth areas | RCW 36.70A.680(2) | RCW 36.70A.680 |
How big can an ADU be in Washington?
A city or county may not cap ADU size below 1,000 square feet. RCW 36.70A.681(1)(f) states a city or county “may not establish a maximum gross floor area requirement for accessory dwelling units that is less than 1,000 square feet.” A jurisdiction may allow more, but 1,000 square feet is the floor it must permit. On roof height, RCW 36.70A.681(1)(g) states a city or county “may not establish roof height limits on an accessory dwelling unit of less than 24 feet,” unless the principal-unit limit is lower.
Does Washington require owner occupancy for an ADU?
No. RCW 36.70A.681(1)(b) states a city or county “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.” Owner occupancy is the rule that the property owner must live in one of the units on the lot, and Washington bars cities and counties from imposing it inside urban growth areas.
Do you need parking for an ADU in Washington?
No off-street parking may be required within one-half mile walking distance of a major transit stop. RCW 36.70A.681(2)(a)(i) bars a city or county from requiring “off-street parking as a condition of permitting development of accessory dwelling units within one-half mile walking distance of a major transit stop.” Outside that half-mile radius, parking caps still apply: one space on lots under 6,000 square feet, and two spaces on lots over 6,000 square feet.
What can local governments still regulate?
Washington law limits how far local rules can go, but cities and counties still administer permitting. Design and dimensional rules may not be more restrictive for ADUs than for principal units: RCW 36.70A.681(1)(h) blocks setback, yard-coverage, tree-retention, entry-door, aesthetic, or design-review rules that are more restrictive than those for principal units. On a very small residential lot of 2,000 square feet or less, a jurisdiction may cap ADUs at two, under RCW 36.70A.681(3). The core ADU definitions, including attached, detached, gross floor area, major transit stop, and owner defined as at least 50 percent ownership, are set in RCW 36.70A.696, as amended by HB 1337 section 2.
When must Washington cities and counties comply?
Compliance is tied to each jurisdiction’s comprehensive plan update cycle, not to a single statewide calendar date. The timing language changed between 2023 and 2025, so the exact trigger depends on which version of the statute you read.
- Current statute, as amended by 2025 c 148 s 4: RCW 36.70A.680(1)(a) requires cities and counties to adopt these requirements “to take effect at the same time as the jurisdiction’s next periodic comprehensive plan update required under RCW 36.70A.130.”
- As originally enacted in 2023: HB 1337 section 3(1)(a) required the rules “to take effect six months after the jurisdiction’s next periodic comprehensive plan update required under RCW 36.70A.130.” The 2025 amendment changed “six months after” to “at the same time as.”
- The Washington Department of Commerce, describing the original 2023 timing, states the bill “requires local governments to update comprehensive plan policies and regulations within six months after the due date of their next GMA periodic update.”
If a city or county has not adopted the required rules, RCW 36.70A.680(1)(b) provides that the state requirements “supersede, preempt, and invalidate any conflicting local development regulations.”
There is no single statewide date, but there is a fixed date for every county. RCW 36.70A.130(5) sets the periodic update deadlines by county, and those deadlines are what the ADU rules ride on:
| Counties (and the cities within them) | Periodic update due | Statute |
|---|---|---|
| King, Kitsap, Pierce, Snohomish | December 31, 2024 | RCW 36.70A.130(5)(a) |
| Clallam, Clark, Island, Jefferson, Lewis, Mason, San Juan, Skagit, Thurston, Whatcom | December 31, 2025 | RCW 36.70A.130(5)(b) |
| Benton, Chelan, Cowlitz, Douglas, Franklin, Kittitas, Skamania, Spokane, Walla Walla, Yakima | December 31, 2026 | RCW 36.70A.130(5)(c) |
| Adams, Asotin, Columbia, Ferry, Garfield, Grant, Grays Harbor, Klickitat, Lincoln, Okanogan, Pacific, Pend Oreille, Stevens, Wahkiakum, Whitman | June 30, 2027 | RCW 36.70A.130(5)(d) |
Two practical notes. Smaller counties (under 50,000 people with slow growth) and small cities (5,000 or fewer with slow growth) in the 2025, 2026 and 2027 groups may comply up to 24 months after their deadline under RCW 36.70A.130(6)(b) and (c). And for Seattle, Tacoma, Everett and the rest of the central Puget Sound counties, the update was due at the end of 2024, so the HB 1337 rules should already be in local code; where they are not, RCW 36.70A.680(1)(b) makes the state rules control over any conflicting local regulation.
What is the difference between HB 1337 and HB 1110?
HB 1110, Engrossed Second Substitute House Bill 1110, Chapter 332, Laws of 2023, is Washington’s middle housing law. It shares the July 23, 2023 effective date with HB 1337 and its core is codified at RCW 36.70A.635, “Minimum residential density.” Both laws operate inside urban growth areas of Growth Management Act jurisdictions, and both use the same “major transit stop” concept and the same half-mile no-parking-mandate rule. HB 1337 guarantees a baseline right to at least two ADUs on single-family lots. HB 1110 layers on a right to more primary units per residential lot by city size and transit proximity.
| City size or location | Base minimum units per lot | Near a major transit stop or with affordability | Statute | Source |
|---|---|---|---|---|
| 25,000 to under 75,000 population | At least two units per lot | At least four units per lot within a quarter-mile walk of a major transit stop, or if one unit is affordable | RCW 36.70A.635(1)(a) | RCW 36.70A.635 |
| 75,000 or more population | At least four units per lot | At least six units per lot within a quarter-mile walk of a major transit stop, or if two units are affordable | RCW 36.70A.635(1)(b) | RCW 36.70A.635 |
| Under 25,000 in a large contiguous UGA | At least two units per lot | Applies where the contiguous UGA includes the largest city in a county over 275,000 | RCW 36.70A.635(1)(c) | RCW 36.70A.635 |
Under RCW 36.70A.635(5), cities “must allow at least six of the nine types of middle housing,” and “may allow accessory dwelling units to achieve the unit density.” Because HB 1110 expressly lets ADUs count toward its unit-density minimums, the two laws stack: a homeowner can pursue ADUs under HB 1337 while the lot’s underlying middle-housing capacity is set by HB 1110. HB 1110 also bars off-street parking mandates for middle housing “within one-half mile walking distance of a major transit stop,” under RCW 36.70A.635(6)(d). HB 1110 compliance follows its own schedule under RCW 36.70A.635(11).
Do Seattle and Tacoma offer pre-approved DADU plans?
Yes. Both cities run pre-approved detached ADU plan programs to speed permitting.
Seattle’s Office of Planning and Community Development runs a pre-approved DADU, or backyard cottage, plan program. Seattle OPCD states these building plans “offer a faster, easier, and more predictable permitting process for creating a DADU,” and that “In most cases, you can get your permit in just 2-6 weeks.” Homeowners can view a gallery of pre-approved designs created by local designers and architects.
The City of Tacoma lets homeowners “Add up to two ADUs on your property.” In Tacoma, “The living area of an ADU is limited to 1,000 square feet, excluding garages and other non-living areas,” and “Parking is not required for ADUs.” Tacoma also offers pre-approved DADU base models.
How Washington compares to other states
Washington’s approach of setting statewide floors on ADU size, parking, and owner occupancy resembles reforms in several other states, though the mechanisms differ. For a contrast, see how Massachusetts ADU law makes ADUs protected uses by right, and how Colorado ADU law handles its own statewide standards. Maine ADU law is the closest East Coast match, with a statewide guarantee that also bans owner-occupancy rules. If you are weighing a smaller structure, our tiny house in a backyard page explains how those rules interact with ADU law. You can browse every state we cover in the ADU laws library.
For what a build actually costs in this state, see our Washington ADU cost guide.
What changed in 2026?
No further statewide Washington ADU changes took effect in 2026 that we could find in the current RCW text as of October 18, 2026. What 2026 does bring is the compliance date for the third group of counties, including Spokane, Benton, Yakima and Walla Walla, whose periodic updates are due December 31, 2026. The operative changes remain HB 1337 and HB 1110, both effective July 23, 2023, along with the 2025 amendment that changed the compliance-timing language in RCW 36.70A.680 from “six months after” to “at the same time as” the next comprehensive plan update.
What changes in 2027?
No further statewide Washington ADU changes are scheduled as of October 18, 2026. The last group of counties reaches its periodic update deadline on June 30, 2027. We recheck the Washington State Legislature each session and will update this page when a new bill is chaptered.
Frequently Asked Questions
How many ADUs can I build on a lot in Washington?
Inside an urban growth area, a city or county must allow at least two ADUs on any lot in a zoning district that permits single-family homes, under RCW 36.70A.681(1)(c). The allowed configurations are one attached plus one detached, two attached, or two detached.
Does Washington require the owner to live on the property to have an ADU?
No. RCW 36.70A.681(1)(b) bars a city or county from requiring the lot owner to live in the ADU or another unit on the same lot. Owner occupancy cannot be imposed inside urban growth areas.
Do I need to provide parking for an ADU in Washington?
Not within one-half mile walking distance of a major transit stop, under RCW 36.70A.681(2)(a)(i). Outside that radius, a city or county may require up to one space on lots under 6,000 square feet and up to two spaces on lots over 6,000 square feet.
What is the largest ADU size a Washington city must allow?
A city or county may not set a maximum ADU size below 1,000 square feet, under RCW 36.70A.681(1)(f). It may allow larger, but 1,000 square feet is the minimum size it must permit.
How much can a city charge in impact fees for an ADU?
Impact fees on an ADU may not exceed 50 percent of the impact fees that would apply to the principal unit, under RCW 36.70A.681(1)(a).
When do Washington cities and counties have to comply with the HB 1337 ADU rules?
Compliance is tied to each jurisdiction’s next periodic comprehensive plan update under RCW 36.70A.130, not a single statewide date. The current statute, as amended in 2025, requires the rules to take effect “at the same time as” that update. As originally enacted in 2023, the rules were to take effect “six months after” it.
What is the difference between HB 1337 and HB 1110?
HB 1337 is Washington’s ADU law, guaranteeing at least two ADUs on single-family lots in urban growth areas. HB 1110 is the middle housing law, requiring two to six primary units per residential lot by city size and transit proximity. HB 1110 lets ADUs count toward its unit-density minimums, so the two stack.
Do Seattle and Tacoma offer pre-approved DADU plans?
Yes. Seattle’s Office of Planning and Community Development offers pre-approved backyard cottage designs and reports permits in about 2 to 6 weeks in most cases. Tacoma offers pre-approved DADU base models and allows up to two ADUs per property.
Sources
- Washington State Legislature, RCW 36.70A.681 (ADUs, limitations on local regulation): https://app.leg.wa.gov/RCW/default.aspx?cite=36.70A.681
- Washington State Legislature, RCW 36.70A.680 (ADUs, local regulation): https://app.leg.wa.gov/RCW/default.aspx?cite=36.70A.680
- Washington State Legislature, RCW 36.70A.635 (minimum residential density, middle housing): https://app.leg.wa.gov/RCW/default.aspx?cite=36.70A.635
- HB 1337 enrolled session law, Chapter 334, Laws of 2023: https://lawfilesext.leg.wa.gov/biennium/2023-24/Htm/Bills/Session%20Laws/House/1337.SL.htm
- HB 1110 enrolled session law, Chapter 332, Laws of 2023: https://lawfilesext.leg.wa.gov/biennium/2023-24/Htm/Bills/Session%20Laws/House/1110-S2.SL.htm
- Washington Department of Commerce, HB 1337 ADU guidance notice: https://www.commerce.wa.gov/uncategorized/30-day-comment-period-hb-1337-draft-accessory-dwelling-unit-adu-guidance/
- Seattle Office of Planning and Community Development, Encouraging Backyard Cottages: https://www.seattle.gov/opcd/current-projects/encouraging-backyard-cottages
- City of Tacoma, Accessory Dwelling Units: https://tacoma.gov/government/departments/planning-and-development-services/permits-and-development-services/residential-permits/accessory-dwelling-units/
- Washington State Legislature, RCW 36.70A.130 (comprehensive plan review schedules): https://app.leg.wa.gov/RCW/default.aspx?cite=36.70A.130
Disclaimer
This page is general information, not legal advice. ADU law changes often and cities and counties apply it differently. Confirm the current rules with your local planning department before you build.