What is the 2026 cost range for a single-story addition scope home addition in Seattle?

$175,000-$400,000, with the median landing near $287,500. The range reflects finish tier and layout complexity within the single-story addition scope; the Seattle labor multiplier of 1.25× vs national median is baked in.

How long does a single-story addition scope home addition take in Seattle?

18-26 weeks of on-site construction for the single-story addition 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?

Matching old siding or brick — if the material is discontinued, the whole elevation has to be reclad. $8-20K unexpected if not scoped up front.

Home addition — Single-story addition Cost in Seattle, 2026

Low

$175K

$175,000

Median

$288K

$287,500

High

$400K

$400,000

Scope timeline: 18-26 weeksSeattle permit window: 8-18 weeksUpdated 2026-04-24

Why single-story addition × Seattle produces this band

A single-story addition scope for a home addition in Seattle combines two price levers: scope and metro labor rates. Single-story additions extend the existing footprint at grade — new foundation section, roof tie-in, and framing that matches the existing structure. No structural work on the original house beyond the tie-in. Against the national median remodel labor rate, Seattle runs 25% above the national median — so the single-story addition 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 single-story addition scope

·Foundation extension (slab or stem-wall to match existing)
·Roof tie-in flashing and structural connection
·Exterior siding match (often requires whole-elevation repaint)
·HVAC extension into the new zone

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 home addition with Baily.

Baily asks the eight questions that determine where your project lands inside the $175K-$400K 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

$175,000-$400,000, with the median landing near $287,500. The range reflects finish tier and layout complexity within the single-story addition scope; the Seattle labor multiplier of 1.25× vs national median is baked in.