An accessory dwelling unit (ADU) is a second, self-contained home on the same lot as your main house — a basement or garage suite (attached, AADU) or a backyard cottage (detached, DADU). In Seattle you can now have two of them on one lot, in any combination: one attached + one detached, two attached, or two detached.
That is the headline change, and it is recent. Seattle brought its code into compliance with Washington’s statewide ADU law (HB 1337) in Ordinance 127211, effective June 30, 2025, consolidating the ADU standards into Seattle Municipal Code 23.42.022. Then Ordinance 127376 took effect January 21, 2026, replacing the interim rules with permanent Neighborhood Residential zoning. If you read ADU advice written before 2025, treat its numbers as stale — several of the old limits below no longer apply.
What you can build, in plain English
- How many: up to two ADUs alongside the main house on a residential lot.
- How big: 1,000 sq ft of gross floor area for a one- or two-bedroom unit, or 1,200 sq ft for three or more bedrooms, under the January 2026 code. (Older guides still quote a flat 1,000 sq ft — the 1,200 sq ft figure for larger units is the newer rule.)
- How tall: a detached unit can reach up to 32 ft in most Neighborhood Residential zones under the January 2026 code, roughly the same height as the main house. Older guides quote 18 ft or 24 ft — those are pre-2026 figures. Confirm the height for your parcel with SDCI before designing.
- Setbacks: 5 ft from the side and rear property lines for a detached unit; 0 ft on an alley under the January 2026 code. Taller portions may need more setback.
- Lot size: there is no longer a minimum lot size for a backyard cottage under the current code — the old 3,200 sq ft minimum was repealed in the 2025 update. The practical limit is whether the cottage fits within your lot’s setbacks and lot coverage.
- Parking: no off-street parking is required for an ADU in Seattle.
- Owner-occupancy: not required. Seattle removed it in 2019 and state law bars cities from bringing it back. You can rent the main house and the ADU(s) at the same time.
- Permit: every ADU needs a building permit through the Seattle Department of Construction and Inspections (SDCI), with site, architectural, structural, and energy plans. Pre-approved designs through the city’s ADUniverse program skip much of design review and can permit faster than a custom design.
What this does not promise
An ADU is not feasible on every lot, and this article is not a feasibility finding for any specific property. Zone, lot coverage, rear-yard coverage, trees, environmentally critical areas, utilities (water, sewer, power, side sewer), and access all matter — and they are parcel-specific. That is why SREG does a manual Development Potential assessment first, rather than an automated calculator. Send the address, lot size, current use, and what you want to build, and we will tell you what is actually possible.
How SREG helps
- Development Potential assessment for your lot (manual, no automated promise)
- Sell Your Development Site, if the lot is worth more as a development site than as a single home
- ADU/DADU, duplex/fourplex, and land acquisition — see the Infill Development Hub
This is general information, not legal or permitting advice. Rules change; confirm current requirements with SDCI before designing or applying for a permit.