As of November 2026: up to two ADUs per lot, attached or detached in any combination; above-ground floor area capped at 1,000 square feet for new ADUs, with conversions of existing space allowed to exceed it; height up to 32 feet in Neighborhood Residential and Residential Small Lot zones; no off-street parking; and no owner-occupancy rule.
Where the rules come from
- Seattle's 2019 reforms removed owner occupancy and parking requirements for ADUs.
- Seattle's 2025 ADU update, in effect around June 30, 2025, raised heights, allowed two-unit configurations and removed limits on entrances and exterior stairs.
- Washington's HB 1337 requires cities to allow at least two ADUs per lot, bars owner-occupancy rules and limits parking requirements.
- Seattle's Neighborhood Residential zoning updates under the One Seattle Plan count ADUs toward the number of homes allowed on a lot.
What still limits a cottage?
- Setbacks and lot coverage for the zone.
- Tree protection.
- Environmentally critical areas such as steep slopes.
- Building and energy codes.
Can an ADU be sold?
HB 1337 says a city cannot prohibit selling a condominium unit separately just because it was built as an ADU. Creating a condominium is its own legal process.
The rules are moving
Seattle has been revising Neighborhood Residential zoning in stages since 2025. Confirm the current code with SDCI before you design.
Sources, as of November 2026: City of Seattle OPCD, One Seattle Plan zoning summaries (2025); The Urbanist, "Seattle eases accessory dwelling restrictions" (July 1, 2025); Washington HB 1337 final bill report (2023). General information, not legal advice.
Updated November 2026