ADU Regulations in Washington (WA)
Washington is rated Easy for ADU construction with an ADU-friendliness score of 96/100 (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 passed HB 1337 in 2023, one of the strongest state ADU laws in the US. All cities planning under the GMA must allow at least 2 ADUs per single-family lot. No owner-occupancy requirements, no parking mandates, no impact fees for small ADUs. Seattle has been a national leader in ADU construction since 2019.
Full ADU Regulations Summary for Washington
| ADUs Allowed | Yes |
| Statewide ADU Law | Yes |
| Maximum ADU Size | 1,000 sq ft |
| Owner Occupancy Required | No |
| Parking Required | No |
| Minimum Setback | 5 ft |
| Permitting Timeline | 60 days max |
| Impact Fees | Exempt for ADUs under 800 sq ft |
| 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) | 96 | Yes | No | No | Easy |
| Vermont (VT) | 95 | Yes | No | No | Easy |
| Arizona (AZ) | 93 | Yes | No | No | Easy |
| California (CA) | 93 | Yes | No | No | Easy |
| Oregon (OR) | 93 | Yes | No | No | Easy |
| Colorado (CO) | 91 | 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 96/100.
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?
The state maximum ADU size in Washington is 1,000 square feet. Local jurisdictions may set lower limits. Some cities also limit ADU size to a percentage of the primary dwelling (e.g., 50%).
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.
Can I rent out my ADU in Washington?
Yes, long-term rentals of ADUs are allowed in Washington. Short-term rentals (like Airbnb) are also generally permitted, though your city may have separate short-term rental regulations.
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. Actual timelines can be affected by plan review backlogs, inspection scheduling, and the complexity of your project. Pre-approved ADU plans, where available, can significantly speed up the process.
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. Remember that local rules may be more specific than state-level regulations.