What an ADU really costs in Seattle in 2025
Honest, line-by-line numbers from our last twelve completed DADUs — design, permitting, hard costs, and the line items most contractors hide.
The headline number
Across our last twelve completed detached ADUs in Seattle, the average all-in cost was $355 per square foot, with a range of $330 to $380. We publish honest, current numbers — not pre-COVID marketing rates that quietly drop foundation, utilities, and finish work.
An honest 700-square-foot DADU in Seattle in 2026, built to current code with mid-tier finishes, lands at roughly $245,000 to $275,000 turn-key. A 1,000 sqft build pencils to $330,000–$380,000 turn-key.
Where the money actually goes
Roughly 8% of total cost is design and engineering. 6% is permitting and city fees. 14% is sitework, foundation, and utilities. 32% is shell — framing, roofing, windows, exterior. 28% is interiors — drywall, flooring, cabinetry, paint. 8% is MEP. The remaining 4% is contingency and overhead.
The line item most homeowners under-budget is sitework. Seattle lots have steep slopes, mature trees, and aging utility services. A reasonable sitework budget for a typical urban lot is $35,000 to $65,000 — not the $10,000 figure you'll sometimes see in marketing.
What drives cost up
Steep grade requiring retaining walls. Detached units more than 50 feet from the street (utility trenching). Older sewer lateral requiring replacement. Stormwater detention requirements on small lots. Removal of any tree six inches in diameter or larger.
We catch these in the feasibility study. A two-week, $2,000 study saves five-figure surprises later.
