ADU rules in Seattle, WA
Yes. Seattle requires a construction addition/alteration permit to build an attached or detached ADU, or a permit to establish use to legalize one.
A lot can have up to two ADUs, and ADUs don't require parking.
Since 2025, ADUs can be up to 1,000 sq ft in every zone.
Rules last checked by a person on October 19, 2026. Guidance only; confirm with Seattle before you build.
Limits
| ADUs per lot | Up to 2 |
|---|---|
| ADU size | 1,000 sq ft max in all zones (2025 rules) |
| Parking | Not required for ADUs |
Good to know
- King County bills a sewer capacity charge on every new ADU; SDCI reports each permit to the county.
- Call 811 at least a few days before you dig footings or post holes. Utility locating is free.
- HOA covenants and deed restrictions can be stricter than city rules. Check yours before you build.
Check your lot
Where to apply
- Seattle Services Portal
- SDCI common projects
- SMC 23.44, Neighborhood Residential
- Phone: (206) 684-8600
Sources
- Seattle SDCI, Accessory Dwelling Unit. Checked 2026-10-19.
Building a detached unit. You need a construction addition / alteration permit... only two of them may be ADUs. Certain standards do not apply to accessory dwelling units such as parking.
- 44 CFR 59.1, definition of development (NFIP). Checked 2026-10-19.
Development means any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures...
- 44 CFR 60.3, floodplain management criteria. Checked 2026-10-19.
Require permits for all proposed construction or other development in the community...
- Seattle Building Connections, ADUs and middle housing (July 2025). Checked 2026-10-20.
Size limits for ADUs have been raised to 1000 square feet across all zones... Setback and height standards have been loosened.