ADU — Attached ADU Cost in Seattle, 2026

Why attached adu × Seattle produces this band

A attached adu scope for a adu in Seattle combines two price levers: scope and metro labor rates. Attached ADUs share a wall with the primary house. Needs its own entrance, fire-rated separation, and (in most jurisdictions) independent utility metering — but shares the site's foundation-extension cost. Against the national median remodel labor rate, Seattle runs 25% above the national median — so the attached adu band comes in 5% below 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 attached adu scope

• ·Fire-rated wall assembly (typically 1-hour, sometimes 2-hour)
• ·Independent utility service drops
• ·Shared-wall sound isolation (STC 50+)
• ·Grading and drainage on the new footprint

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 $214K-$451K 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 attached adu scope adu in Seattle?

  $213,750-$451,250, with the median landing near $332,500. The range reflects finish tier and layout complexity within the attached adu scope; the Seattle labor multiplier of 1.25× vs national median is baked in.

• How long does a attached adu scope adu take in Seattle?

  22-32 weeks of on-site construction for the attached adu 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?

  Utility separation surprises — some markets require a new electrical service drop from the street, not just a sub-panel. Utility-company coordination adds 6-10 weeks and $8-14K.