How our Seattle ADU costs compare to AARP's national data
AARP's ADU research is the most-cited benchmark in the country. Here is how Puget Sound numbers stack up.
AARP's national picture
AARP's ADU resources cite typical national construction costs of $100,000 to $300,000 for a new detached ADU, depending heavily on region, size, and finish level.
Their data also documents who is building ADUs and why — and aging-in-place use cases account for a large share of the demand.
Why Seattle runs higher
Three drivers push Puget Sound costs above the AARP national midpoint. Labor — Seattle prevailing trade wages run 25–40% above national medians. Stormwater and tree retention — Seattle's regulatory cost on a typical lot is $15,000–$30,000 above the national norm. Energy code — WA 2021 is among the strictest in the country and adds $10,000–$25,000 to a typical DADU envelope.
Net effect: AARP's national midpoint of roughly $180,000 maps to roughly $290,000–$340,000 for a comparable Seattle build.
Where Seattle saves
No impact-fee for ADUs in most NR zones. Strong rental market — average DADU rents 25–35% above national medians — which collapses payback periods despite higher build cost. And the SDCI Pre-Approved DADU program, which has no national equivalent in most cities.
