ADU — Detached 1-bedroom Cost in Seattle, 2026

Why detached 1-bedroom × Seattle produces this band

A detached 1-bedroom scope for a adu in Seattle combines two price levers: scope and metro labor rates. A detached 1-bedroom ADU is the category anchor: 500-750 SF standalone unit with a full kitchen, full bath, and bedroom. Separate foundation, roof, MEP, and entrance. Against the national median remodel labor rate, Seattle runs 25% above the national median — so the detached 1-bedroom band comes in at the mid-range, then scales 25% above the national median on top of that. Permit timelines for residential work in Seattle typically run 8-18 weeks, which is the window you plan your design and decision sequence against. This page gives you the actual 2026 Seattle bands for this exact combination, plus the three cost drivers Baily asks about first when scoping the project live.

What drives cost for a detached 1-bedroom scope

• ·New foundation (slab-on-grade or stem-wall)
• ·New roof system and weather envelope
• ·Full MEP rough-in from the street (water, sewer, gas, electric)
• ·Site prep, grading, drainage

What makes Seattle different

• Labor — WA LNI-licensed trades required on most work — contractor registration verifiable via online lookup.
• Labor — Wet-season (Oct-Apr) labor premium for exterior trades (roofing, siding, decks).
• Labor — Urban Forestry and tree-retention requirements add arborist labor on some remodels.
• Material — Green building incentives favor FSC-certified lumber, heat-pump HVAC, and higher-R insulation — 5-10% material uplift for Energy Star targets.
• Material — Port of Seattle is a primary entry for Asian-sourced finishes (tile, fixtures) — generally competitive.

Seattle rules that affect this scope

• Seattle DCI runs SDCI plan review — typical residential turnaround 8-18 weeks depending on Land Use review.
• Seattle Energy Code (SEC 2021) exceeds IECC — mandatory heat-pump water heater and HVAC on most new work.
• Design Review required on most multi-family remodels above the small-project threshold.
• Shoreline Management Act affects lakeside and riverside parcels (Lake Washington, Lake Union, Duwamish) — adds 8-14 weeks.

Scope your adu with Baily.

Baily asks the eight questions that determine where your project lands inside the $225K-$475K band for Seattle and hands the scoped brief to one licensed builder.

Related pages

Questions LA homeowners actually ask

• What is the 2026 cost range for a detached 1-bedroom scope adu in Seattle?

  $225,000-$475,000, with the median landing near $350,000. The range reflects finish tier and layout complexity within the detached 1-bedroom scope; the Seattle labor multiplier of 1.25× vs national median is baked in.

• How long does a detached 1-bedroom scope adu take in Seattle?

  26-40 weeks of on-site construction for the detached 1-bedroom scope, plus 8-18 weeks of permit review in Seattle if the scope requires a permit. Cosmetic-only refreshes often skip permits; anything touching plumbing, electrical, or structural does not.

• What is the single biggest budget-overrun risk for this scope?

  Sewer connection — if the existing lateral runs under the new foundation line, a new lateral cut plus utility easement and right-of-way bond can add $12-22K.