Addition costs, Seattle and the Eastside
What a Home Addition Costs in Seattle
$350 to $650 per square foot in 2026. A 400 sq ft primary suite runs $175,000 to $280,000 and a full second story runs $350,000 to $600,000 and up. Here is every type, and what actually moves the number.
The first feasibility check is free and usually takes about a day.
Free feasibility check
Send the address and what you have in mind. David replies within one business day with what the site and the code are likely to allow.
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Read this first
Type matters more than size
Cost per square foot on an addition is not one number, it is five, because the types are genuinely different projects. A rear bump out and a full second story share almost nothing beyond needing a permit.
Small additions also cost more per square foot than large ones, which surprises people. A kitchen or a bathroom concentrates the expensive parts of a house into very little floor area, so the rate goes up as the size comes down.
Work from the type first and the size second. The estimator below is built that way for the same reason.
Estimate
Estimate your addition
Pick the type and the size. These are the same figures as the table below, which is now the single set of numbers this site publishes for additions.
A planning range for construction, not a quote. Architecture fees come on top: a fixed fee at Piper Cole, or roughly 8% to 15% of construction at percentage-priced firms, and a second story also carries structural reinforcement where the existing foundation needs it. Call David on 425 753 6452 about your own house.
Projects
Selected work
Real houses, real constraints. Photographs and drawings from the practice.
The numbers
Home addition cost by type, Seattle and the Eastside 2026
These are now the only addition figures this site publishes. Several older pages quoted different numbers for the same work, and they have been pointed here.
| Type | Per sq ft | Typical total |
|---|---|---|
| Ground floor or rear addition | $350 to $550 | $105,000 to $275,000 |
| Primary suite addition, 400 sq ft | $440 to $700 | $175,000 to $280,000 |
| Kitchen addition, 250 sq ft | $480 to $800 | $120,000 to $200,000 |
| Family or great room addition, 300 sq ft | $400 to $665 | $120,000 to $200,000 |
| Partial second story | $400 to $600 | $200,000 to $450,000 |
| Full second story | $350 to $550 | $350,000 to $600,000 and up |
Swipe the table sideways to see every column
Construction cost. Architecture fees add a fixed fee at Piper Cole (roughly 8% to 15% at percentage-priced firms), structural engineering adds $4,000 to $12,000, and permit fees run roughly 1.5% to 2.5% of project valuation. Budget a 15% contingency on any addition, because you are building onto something that already exists.
What moves it up
The five things that push an addition to the top of its range
None of these are finish decisions. They are conditions, and four of the five are knowable before you design anything.
- Structural complexity. Load bearing changes, a hillside site, or an existing foundation that cannot carry what you want to put on it.
- Site access. A tight lot where deliveries and staging are difficult costs real labor hours, every day, for the whole build.
- Geotechnical conditions. Poor soils, a high water table or a landslide prone slope all add engineering before they add construction.
- Removing the roof. Any second story means temporary weather protection, a new roof over the whole footprint, and usually the family moving out.
- Finish level. The one genuine decision on this list. Bathrooms and kitchens carry most of it.
What brings it down
And the one thing people get wrong about location
A simple rectangular footprint, good site access and standard finishes will keep any addition at the bottom of its range. That is most of the lever you have.
On location, be careful with what you read. Costs vary between Seattle, Kirkland, Bellevue and Redmond, but they vary by site condition, lot access and which jurisdiction is reviewing your permit, not by a flat percentage attached to the town. Anyone quoting you a clean citywide discount or premium is guessing.
What does differ measurably by city is permit process and review time, and on a long project time is money in a way a per square foot table never shows.
Beyond construction
The costs that are not in the contractor’s number
The construction figure is the one everybody quotes and the one that is never the whole project. Plan for these separately.
- Architecture. A fixed fee at Piper Cole; roughly 8% to 15% of construction at percentage-priced firms for full service, covering schematic design through construction administration.
- Structural engineering. $4,000 to $12,000 depending on what is being changed.
- Permit and city fees. Roughly 1.5% to 2.5% of project valuation, plus any survey.
- Contingency. 15% minimum. You are building onto an existing house and the walls usually have something to say once they are opened.
- Temporary housing on a second story. Not in anybody’s bid, and it arrives at the worst possible moment.
Questions
Addition costs, answered
Not answered here? Call David, most calls are answered the same day.
How much does a home addition cost in Seattle in 2026?
$350 to $650 per square foot depending on type. A 400 sq ft primary suite is $175,000 to $280,000, a 250 sq ft kitchen addition is $120,000 to $200,000, and a full second story is $350,000 to $600,000 and up. Architecture fees add a fixed fee at Piper Cole, or roughly 8% to 15% of construction at percentage-priced firms, on top.
Why do small additions cost more per square foot?
Because a kitchen or a bathroom concentrates the expensive parts of a house, plumbing, cabinetry, tile and appliances, into very little floor area. A large family room spreads the same kind of cost across more square feet, so the rate falls even though the total rises.
Is it cheaper to build an addition in Kirkland than in Seattle?
Not by a reliable flat percentage, whatever you may read. The real differences come from site conditions, lot access and which jurisdiction reviews your permit. What does differ measurably by city is permit process and review time, and on a long project that is a real cost.
What should I budget beyond construction?
Architecture at a fixed fee (roughly 8% to 15% of construction at percentage-priced firms), structural engineering at $4,000 to $12,000, permit and city fees at roughly 1.5% to 2.5% of valuation, and a 15% contingency minimum. On a second story, add temporary housing.
Do I need an architect for a home addition in Washington?
Not by law. RCW 18.08.410 exempts single family homes and buildings of four units or fewer from the stamp requirement. You do need permitted drawings and structural engineering. Most people hire an architect because joining new work to an existing house is a design problem.
How much contingency should I carry?
15% as a floor on any addition, and more if the house is pre 1980 or the foundation is an unknown. You are building onto something that already exists, and the surprises are inside the walls.
Keep reading
The rest of the picture
- Second story addition cost in SeattleThe most complex addition type, in full, with its own estimator
- Home addition architectWhat the design work involves, by type
- Architect fees in SeattleThe 8% to 15% market range explained, with a calculator for your own budget
- Lot coverage, setbacks and FAR explainedWhether you can build out at all before you price building out
- Whole house remodel cost, EastsideWhen the addition is really a full remodel
- Seattle building permit guideReview times, fees and what the process involves
Keep reading
Related guides
Are second story additions worth it in Seattle and on the Eastside?
Second stories run $350 to $550 per sq ft on the Eastside, and foundation work adds $30k to $100k. When going up pays off, and when it does not.
Room addition cost in Kirkland and the Eastside
Bedroom additions $85k to $165k, primary suites $220k to $400k in Kirkland. Real 2026 ranges, permit timelines and where the money goes.
Second story addition over a garage
Building over an attached garage is often the cheapest square footage you can add. Real 2026 Seattle costs, the slab problem, and when it works.
Next step
Get the number for your house
The estimator gives you a planning range by type. A thirty minute call gives you a real one, including the structural and site questions that decide where inside the range you land. Call David Meade directly.
Free feasibility check
Send the address and what you have in mind. David replies within one business day with what the site and the code are likely to allow.