Seattle ADU rules
On most Seattle house lots you can add two ADUs — attached, detached or one of each — each up to 1,000 square feet, with no parking and no owner-occupancy requirement.
Seattle ADU rules at a glance
| RULE | SEATTLE | STATE FLOOR |
|---|---|---|
| Number of ADUs | 2 per lot — attached, detached or one of each | 2 per lot |
| Maximum size | 1,000 sq ft per ADU | ≥ 1,000 SF |
| Height, detached | Scales with lot width; roof-pitch allowance on top | ≥ 24 FT |
| Setbacks, detached | 5 ft side and rear; reduced along an alley | — |
| Parking | Not required | NONE NEAR TRANSIT |
| Owner occupancy | Not required | NOT ALLOWED |
| Sold separately | Allowed through a condominium | MUST BE ALLOWED |
Getting a permit in Seattle
ADU permits go through the Seattle Department of Construction and Inspections (SDCI). Utility work is permitted separately with Seattle Public Utilities and Seattle City Light.
- 01Pre-submittal coachingOptional meeting to confirm zoning, setbacks and lot constraints before design is final.[WEEKS]
- 02Application & intakePlans, site plan and supporting documents are uploaded through the Seattle Services Portal.[WEEKS]
- 03Plan review & correctionsZoning, structural and energy reviewers issue corrections; each round of answers restarts their clock.[WEEKS]
- 04Permit issuedFees are paid and the permit is issued; side sewer and electrical permits follow.[WEEKS]
- 05Inspections & finalFoundation, framing, mechanical and final inspections, then the unit can be occupied.[MONTHS]
What the city charges
What’s different in Seattle
Seattle ADU questions
Can I build two ADUs on one Seattle lot?
Yes. Seattle allows two ADUs on a lot where a house is allowed — two attached, two detached, or one of each, as state law requires. Each can be up to 1,000 square feet.
Do I have to live on the property?
No. Seattle has no owner-occupancy requirement, and Washington law no longer lets cities impose one on ADUs.
Do I need to add off-street parking?
No. Seattle does not require a parking space for an ADU.
Can I sell the ADU separately?
Washington requires cities to allow ADUs to be sold as condominium units. That takes a condo declaration, a survey and lender review on top of the building permit.
What makes a Seattle permit faster?
A complete first submittal and a pre-approved DADU plan. Most delay comes from correction rounds, so a full site survey and clear utility plan up front matter most.
