What is the 2026 cost range for a interior conversion scope adu in Seattle?

$112,500-$237,500, with the median landing near $175,000. The range reflects finish tier and layout complexity within the interior conversion scope; the Seattle labor multiplier of 1.25× vs national median is baked in.

How long does a interior conversion scope adu take in Seattle?

14-20 weeks of on-site construction for the interior conversion 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?

Existing slab without a vapor barrier — vapor-pressure moisture leads to flooring failure and mold if not re-poured or sealed. Adds $4-8K.

ADU — Interior conversion Cost in Seattle, 2026

Low

$113K

$112,500

Median

$175K

$175,000

High

$238K

$237,500

Scope timeline: 14-20 weeksSeattle permit window: 8-18 weeksUpdated 2026-04-24

Why interior conversion × Seattle produces this band

A interior conversion scope for a adu in Seattle combines two price levers: scope and metro labor rates. Interior-conversion ADUs repurpose an existing enclosed space (attached garage, basement) without adding foundation or framing footprint. Faster and cheaper than a detached build. Against the national median remodel labor rate, Seattle runs 25% above the national median — so the interior conversion band comes in 50% 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 interior conversion scope

·Insulation and conditioned-space HVAC tie-in
·Egress window cutting and well install
·Kitchen and bath rough-ins (often net-new to the space)
·Legal separation from the primary unit

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 $113K-$238K band for Seattle and hands the scoped brief to one licensed builder.

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Seattle remodel cost hub →Back to all cost guides →Ask Baily about your project →

Questions LA homeowners actually ask

$112,500-$237,500, with the median landing near $175,000. The range reflects finish tier and layout complexity within the interior conversion scope; the Seattle labor multiplier of 1.25× vs national median is baked in.