Yes. As of November 2026, Seattle allows up to two ADUs per lot, and Washington's HB 1337 requires cities to allow two. They can be one attached and one detached, two attached, or two detached, including two detached units built like a duplex.
Common combinations
- A basement unit plus a backyard cottage.
- A garage conversion plus an in-home unit.
- Two detached units sharing a wall.
How do they count?
Under Seattle's 2025 Neighborhood Residential rules, ADUs count toward the number of homes allowed on the lot. On most lots that leaves room for two ADUs; confirm the total for your parcel.
Sources, as of November 2026: City of Seattle OPCD zoning summaries (2025); Washington HB 1337 final bill report. General information, not legal advice.
Updated November 2026