Home Remodeling in
Seattle, WA

In May 2025 the City Council adopted Ordinance 127211 to bring Seattle's ADU rules in line with HB 1337. In June it adopted Ordinance 127219, interim changes to the Neighborhood Residential zones, the former single-family zones, to allow middle housing citywide under HB 1110. Both took effect on 30 June 2025.

The practical result is that some lots are now large enough for four or more homes, but only two of those can be ADUs. The rest are treated as principal homes, with their own standards, so it is worth working out which route suits your lot before drawing anything.

ADUs are exempt from several requirements that apply to other housing, including parking, street improvements and the city's Mandatory Housing Affordability contribution. They still have to meet the zone's height and location standards.

Through ADUniverse, the city's ADU website, Seattle offers a gallery of pre-approved backyard cottage designs by local architects and designers. The city says most of them can be permitted in two to six weeks, much faster than a custom design.

The gallery is being refreshed. The city's planning office ran a call for new ADU and middle housing designs, with submissions extended to 15 July 2026, so more plans are on the way.

Plenty of Seattle houses already have a basement apartment or converted space that was never permitted. The city has a route for that: a construction permit to establish use, which brings the existing unit into the record as an ADU.

An ADU created inside the house or as a new backyard cottage uses a construction addition or alteration permit instead. Older owner-occupancy covenants on ADUs can also be released. SDCI takes questions on (206) 684-8600.