Seattle ADU guide
Building an ADU in Seattle: what fits on your lot and what it costs
Seattle allows accessory dwelling units — backyard cottages and attached suites — on most residential lots. What actually fits depends on your lot, its zone, and rules that set coverage, height, setbacks and unit size. This page shows the rules we check against and planning-grade cost bands, with every assumption stated.
The zoning rules we check against
Rules version 2026.08-unverified for Seattle, WA. These are the exact figures our feasibility check uses — shown with our confidence level so you know how far to trust them. Verify with the city before relying on any figure; lot-specific conditions can differ.
| Zone | Lot coverage | Height | Setbacks (front / side / rear) | Unit area cap | Confidence |
|---|---|---|---|---|---|
| NR1 | 35% | 16 ft | 20 / 5 / 5 ft | 1000 sq ft | low · source |
| NR2 | 35% | 16 ft | 20 / 5 / 5 ft | 1000 sq ft | low · source |
| NR3 | 35% | 16 ft | 20 / 5 / 5 ft | 1000 sq ft | low · source |
Height limits vary with roof form and lot slope; unit-area caps have exclusions. A licensed professional must confirm what applies to your lot.
What a backyard home could cost
Planning bands at a standard finish level, never single numbers — real bids set real prices.
Studio ADU
400 sq ft
$124k–$216k
$312–$539 per sq ft
One-bedroom ADU
600 sq ft
$187k–$323k
$312–$539 per sq ft
Two-bedroom ADU
960 sq ft
$299k–$517k
$312–$539 per sq ft
What these bands assume
- Estimated from per-square-foot construction bands (version v1-2026-08) at the standard finish level, applied to 400 sq ft of conditioned floor area.
- A regional cost factor of 1.2 is applied, taken from the jurisdiction on record; its accuracy is not yet verified.
- The high end of the band includes a 15% allowance for site conditions, utility work and construction overruns.
- Excluded: land, design fees, permit fees, utility connection charges, and site work beyond the allowance above.
- This is an early-stage planning band, not a bid. A licensed professional must price the actual design.
Seattle's climate shapes the build
Seattle sits in climate zone 4C (Marine): mild temperatures, long wet seasons, real moisture management requirements. That drives the insulation, glazing and moisture strategy a backyard home here should use — guidance the Materials step of our flow is built around.
See what fits on your Seattle lot
Common questions
How much does an ADU cost to build in Seattle?+
For planning purposes we show ranges, never single numbers. At a standard finish level our current bands put a studio-sized backyard home roughly in the low-to-mid six figures and larger two-story units higher — see the bands above with every assumption listed. Land, design fees, permit fees and utility connections are excluded, and only real bids set real prices.
What are the ADU setback requirements in Seattle?+
The rules we currently check against for neighborhood residential zones include front, rear and side setbacks along with lot-coverage, height and unit-area limits — the exact figures and their sources are in the table above, marked with our confidence level. Verify with the city before relying on them; lot-specific conditions can differ.
Can I build a backyard cottage on my lot?+
Most Seattle residential lots allow accessory dwelling units, but what fits depends on your lot area, shape and zone. Enter your ZIP and lot basics below to get a feasibility estimate with every assumption stated — then a licensed professional must confirm what can actually be built.
Is this an official determination?+
No. These are early-stage planning estimates against an estimated envelope. Only the jurisdiction decides what you may build, and a licensed professional must confirm anything before you rely on it.