How much does a home addition cost per square foot in Seattle?

Ground-level additions in the Seattle area typically run $250–$450 per square foot in 2026, all-in — design, permits, foundation, framing, systems and finishes. Second-story additions usually cost more per foot because of the structural work required below.

Is an addition cheaper than moving?

Often, yes. Selling and buying costs roughly 8–10% of your home's value in fees and excise tax before you've gained a single square foot — on an $900k house that's $70k+ gone. A well-scoped addition puts the entire budget into space you keep.

How long does a home addition take?

Plan on 2–4 months of design and permitting, then 3–6 months of construction depending on size and complexity. Second stories run longer than ground-level additions. A written schedule comes with the estimate.

Do I need an architect for an addition?

You need stamped drawings for the permit, and usually structural engineering. Whether that's an independent architect or a design-build package depends on the project — for most straightforward additions, design-build is faster and keeps one party accountable for both drawings and cost.

Field Notes · August 3, 2026 · 7 min read

How Much Does a Home Addition Cost in the Seattle Area?

Real 2026 home addition costs for Seattle, the Eastside and Snohomish County: per-square-foot ranges, ground-level vs. second-story, what drives the number, and how to budget.

Completed home addition by ENF General Contracting in the Seattle area

An addition is the biggest remodel most homeowners ever take on, and the pricing feels opaque from the outside. It doesn’t need to be. Here’s how addition budgets actually break down in the Seattle area in 2026 — the same way we walk clients through it at the kitchen table.

The honest per-square-foot answer

Ground-level additions across Seattle, the Eastside and Snohomish County generally land between $250 and $450 per square foot, all-in. A 400 sq ft family-room addition, then, is realistically a $100k–$180k project. The spread is wide because the number includes everything: design and engineering, permits, excavation and foundation, framing, roofing, siding, windows, electrical, heating, insulation, drywall and finishes.

Second-story additions run higher per foot — commonly $350–$550+ — because the existing structure has to carry the new load. That means engineering, often new footings or wall reinforcement, a new roof, and a stretch where your house needs serious weather protection mid-build. What you get in exchange: doubled bedroom count with zero yard lost, which is why they’re so popular in view neighborhoods like Kirkland.

What actually drives the number

Foundation and site. Flat lot with easy access is the cheap end. Slopes, tight side yards, tree roots and bad soil all add real money before the addition is even out of the ground.
What the room is. A bedroom is drywall and carpet; a kitchen extension or a bathroom brings plumbing, tile and cabinetry with it. Wet rooms cost more per foot than dry rooms, every time.
Matching the existing house. Tying a roofline in cleanly, matching siding and windows so the addition doesn’t look bolted on — that’s craft, and it’s worth paying for. Additions that skip it show it forever.
Systems capacity. Many older panels and furnaces can’t absorb 400 more square feet. If an upgrade is needed, you want it scoped in the estimate — not discovered in month three.

The permit reality

Every addition needs a building permit, stamped drawings and usually structural engineering. Timelines vary sharply by city — weeks in most Snohomish County jurisdictions, months at Seattle’s SDCI. We cover the whole process in our addition permits guide, and on our jobs we handle it start to finish as part of the build.

Addition, ADU — or move?

If the new space could ever house a parent, an adult kid or a renter, price the ADU version of the same square footage — the ADU cost guide covers it. And if you’re weighing an addition against selling, run the math in remodel vs. moving first — transaction costs change the answer more than most people expect.

Get a real number for your house

Per-square-foot ranges are for orientation; your lot, your foundation and your roofline set the actual price. The free in-home estimate gets you a real number and an honest read on whether the addition you’re imagining is the right move for the house.

Price your addition →

Frequently asked questions

How much does a home addition cost per square foot in Seattle?: Ground-level additions in the Seattle area typically run $250–$450 per square foot in 2026, all-in — design, permits, foundation, framing, systems and finishes. Second-story additions usually cost more per foot because of the structural work required below.
Is an addition cheaper than moving?: Often, yes. Selling and buying costs roughly 8–10% of your home's value in fees and excise tax before you've gained a single square foot — on an $900k house that's $70k+ gone. A well-scoped addition puts the entire budget into space you keep.
How long does a home addition take?: Plan on 2–4 months of design and permitting, then 3–6 months of construction depending on size and complexity. Second stories run longer than ground-level additions. A written schedule comes with the estimate.
Do I need an architect for an addition?: You need stamped drawings for the permit, and usually structural engineering. Whether that's an independent architect or a design-build package depends on the project — for most straightforward additions, design-build is faster and keeps one party accountable for both drawings and cost.

Written by

Eduard — Owner & General Contractor

Eduard owns ENF General Contracting and has built and remodeled homes across Everett, Seattle & the Eastside for over a decade. Licensed, Bonded & Insured. He runs every ENF project personally — and writes these guides from the job site. More about Eduard · (425) 217-9921