ADU Regulations in Washington (WA)
Washington is rated Easy for ADU construction with an ADU-friendliness score of 83/100 (Very ADU-Friendly). Washington has a statewide ADU law that sets baseline rules across all municipalities. No owner-occupancy requirement. No additional parking required for ADUs. Washington's HB 1337 (2023, codified at RCW 36.70A.681) requires cities and counties planning under the GMA to allow at least two ADUs per lot in urban growth areas. Cities may not cap ADU size below 1,000 sq ft — that's a floor on local limits, not a state maximum. No owner-occupancy requirements; impact fees capped at 50% of the principal unit's. Parking: cities may still require up to 1 space on lots under 6,000 sq ft (2 on larger lots) outside half-mile transit zones.
Full ADU Regulations Summary for Washington
| ADUs Allowed | Yes |
| Statewide ADU Law | Yes |
| Maximum ADU Size | Set locally |
| Owner Occupancy Required | No |
| Parking Required | No |
| Minimum Setback | 5 ft |
| Permitting Timeline | 60 days max |
| Impact Fees | Capped at 50% of the impact fees that would apply to the principal unit (RCW 36.70A.681) |
| Long-Term Rental Allowed | Yes |
| Short-Term Rental (Airbnb) Allowed | Yes |
| Ease of Building | Easy |
ADU Regulations by City in Washington
ADU rules can vary by city even within the same state. While Washington's statewide law sets minimum standards, some cities have additional local provisions. Below are ADU details for 4 cities in Washington.
Seattle
ADUs allowed by right. Seattle permits 2 ADUs per lot (1 attached + 1 detached). No owner occupancy, no parking. Pre-approved DADU plans available. Nation's highest per-capita ADU construction rate.
Tacoma
ADUs permitted on all residential lots. Tacoma allows 2 ADUs per lot per state law. Streamlined permitting.
Spokane
ADUs allowed per state law. Spokane updated zoning in 2024 for full compliance with HB 1337.
Bellevue
ADUs permitted on single-family lots. Bellevue allows both attached and detached ADUs. No additional parking.
How Washington Compares to Other States
Below is a comparison of Washington with five states that have similar ADU-friendliness scores. This comparison helps illustrate where Washington stands nationally.
| State | Score | Statewide Law | Parking Req? | Owner Occ? | Ease |
|---|---|---|---|---|---|
| Washington (WA) | 83 | Yes | No | No | Easy |
| Arkansas (AR) | 83 | Yes | No | No | Easy |
| Massachusetts (MA) | 85 | Yes | No | No | Easy |
| Arizona (AZ) | 86 | Yes | No | No | Easy |
| Maine (ME) | 80 | Yes | No | No | Easy |
| Minnesota (MN) | 88 | Yes | No | No | Easy |
Frequently Asked Questions About ADUs in Washington
Can I build an ADU in Washington?
Yes, ADUs are allowed in Washington. Washington has a statewide ADU law that requires local jurisdictions to permit ADUs on residential lots. This means your city cannot outright ban ADUs. The ease of building is rated "Easy" with a friendliness score of 83/100.
Do I need to live on the property to have an ADU in Washington?
No, Washington does not require owner occupancy for ADUs. You can rent out both the main house and the ADU, which maximizes rental income potential.
How big can an ADU be in Washington?
Washington does not set a statewide maximum ADU size — this is determined by your local jurisdiction. Common local limits range from 600 to 1,200 square feet. Check your city's zoning code for the specific maximum.
Do I need extra parking for an ADU in Washington?
No — Washington does not require additional parking for ADUs. This is a significant advantage, as parking requirements are often the biggest barrier to ADU construction on smaller lots.
How long does it take to get an ADU permit in Washington?
The permitting timeline in Washington is: 60 days max. The statewide law sets maximum review periods that cities must comply with.
Does Washington have a statewide ADU law?
Yes. State law sets the floor that Washington cities must meet: no statewide size cap, no owner-occupancy mandate, no added parking, with permitting handled as: 60 days max. Cities can be more permissive than the state baseline but generally not stricter.
Check ADU Rules for Your City
Use our ADU regulations lookup tool to find the specific rules for your city in Washington, or browse all 50 states to compare ADU regulations across the country.