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House Addition Cost in Seattle, Washington
The median declared cost of a house addition in Seattle, Washington is $150,000, based on 884 residential building permits issued in the last 24 months.
Updated November 2026 · next refresh December 2026 · computed from permits issued through November 2026.
| Median declared cost | $150,000 |
|---|---|
| Typical range (middle half) | $64,982 – $250,000 |
| Full spread (10th–90th percentile) | $25,000 – $387,720 |
| Permits in sample | 884 |
| Period covered | 2024-11-12 to 2026-11-12 |
| Source | City of Seattle building permits (open data) |
How to read this number
These are declared permit values — the job value the applicant reported to the City of Seattle when the permit was filed. Declared values are a real-world sample of what these projects get filed at, but they are not final invoices: applicants have no reason to inflate them, and final costs often run higher once the work is done. Use the median as a planning anchor and the range to see how far real projects spread.
One more caveat: a permit value covers the whole permitted job. When a house addition is filed together with other work, the single declared figure covers all of it — one reason the top of the range runs high.
An addition is three projects wearing one permit. It starts below ground with a foundation pour — the concrete slab calculator does that math in yards and bags — it is topped by a roof, priced with the roofing calculator, and everything between them keys off the new floor area, which the square footage calculator measures in a minute.
How the costs spread
Half of all permits in the sample declared between $64,982 and $250,000. The full spread is wider: the cheapest 10% declared under $25,000, and the most expensive 10% declared over $387,720.
Median cost by ZIP code
The same project, priced by neighborhood. ZIP medians use the same 24-month sample as the headline figure; a ZIP is listed only when at least 30 of its permits are in the sample.
| ZIP code | Permits | Median declared cost |
|---|---|---|
| 98117 | 88 | $120,500 |
| 98103 | 85 | $178,838 |
| 98115 | 85 | $165,000 |
| 98199 | 55 | $150,000 |
| 98112 | 49 | $150,000 |
| 98125 | 49 | $132,089 |
| 98118 | 45 | $114,613 |
| 98116 | 44 | $152,869 |
| 98136 | 39 | $150,000 |
| 98105 | 36 | $162,996 |
| 98122 | 35 | $168,194 |
How this project gets permitted in Seattle
Seattle permit descriptions name the review path each job actually took. Of the 873 permits in this sample whose descriptions name it, 78% went through full plan review (193 STFI vs 680 full plan review). STFI is the city's faster track for work that does not need a full plan check; structural work goes through plan review.
The permit itself is a small line item. SDCI charges by declared value (Development Fee Index, Seattle Municipal Code 22.900D.010, Tables D-1 and D-2): at this project's median declared value of $150,000, the building permit and plan review on the full plan review path come to about $4,645, including the 5% technology fee and the $6.50 state building-code fee. Across the middle half of declared values ($64,982–$250,000), the same fees run about $2,782–$6,273. Trade permits (electrical, plumbing, mechanical) are charged separately.
Trend
Median declared cost was $100,000 in 2024 (45 permits) and $150,000 in 2026 (387 permits) — up about 50% across the two years.
| Year issued | Permits | Median declared cost |
|---|---|---|
| 2024 | 45 | $100,000 |
| 2025 | 452 | $146,266 |
| 2026 | 387 | $150,000 |
Recent house addition permits in Seattle
Anonymized examples from the sample — work description and declared value as filed, no addresses or names.
| Issued | Permit description | Declared cost |
|---|---|---|
| Nov 2026 | Construct substantial alterations and additions to existing one family dwelling per plan | $276,268 |
| Nov 2026 | Construct addition & alterations to one-family dwelling subject to field inspection (STFI) | $36,182 |
| Nov 2026 | Allow housing for new stacked dwelling unit within an existing structure per land use code. Construct addition and alterations for a two fa… | $150,000 |
| Nov 2026 | Construct addition to one family dwelling subject to field inspection STFI. | $20,000 |
| Nov 2026 | Construct addition to single family residence per plan. | $29,078 |
| Nov 2026 | Construct substantial alterations and addition to existing one family residence subject to field inspection (STFI). | $70,000 |
Methodology
We pull every issued residential building permit the City of Seattle publishes through its open-data portal (Socrata), keep permits from the last 24 months with a declared value above zero, and classify each one by the project its description names. Records above $2,000,000 and statistical outliers (outside 1.5× the interquartile range) are excluded before the median is computed. Every classifier rule set is spot-checked by hand against 20 sampled permits before a project type is published. This page is refreshed monthly from the permit archive.
Frequently asked questions
How much does a house addition cost in Seattle, Washington?
Based on 884 Seattle, Washington building permits issued in the last 24 months that included a house addition, the median declared cost was $150,000. The middle half of permits declared between $64,982 and $250,000.
Where do these numbers come from?
They are computed from building permits published by the City of Seattle through its open-data portal. Each permit carries a declared job value reported by the applicant. We filter to residential permits, classify them by the project described, drop outliers, and publish the median and the middle-half range.
Why might my quote be higher than the permit median?
The declared value is what the applicant reported to the city when the permit was filed, and final project costs often run higher. Scope also varies widely: a permit value covers the whole permitted job, so a remodel combined with other work declares one combined figure. Treat the median as a planning anchor, not a bid.
What does the declared value on a house addition permit cover?
Whatever work the permit describes — for combined projects that can include rooms or work beyond the headline project. That is why we publish the range alongside the median: it shows how wide real declared costs spread for this project type in Seattle.
Does a house addition in Seattle need plan review?
Most take the full plan review path: 78% of the 873 permits in our sample whose descriptions name the review path went that way (193 STFI, 680 full plan review). SDCI decides the path at intake — non-structural work usually qualifies for STFI, structural work does not.
How much is the permit fee for a house addition in Seattle?
SDCI charges by declared value. At this project's median declared value of $150,000, the building permit plus plan review on the full plan review path comes to about $4,645, including the 5% technology fee and the $6.50 state fee (Seattle Municipal Code 22.900D.010). Trade permits are extra.
Plan the work
Concrete Slab Calculator
Cubic yards and bag counts for slabs and pads, with ready-mix vs bag cost comparison.
CalculatorDrywall Calculator
Drywall sheets for any room, plus the mud, tape and screws to finish them — at published estimating rates.
CalculatorRoofing Calculator
Shingle squares and bundles from footprint and pitch, with underlayment and ridge cap.
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Cost in SeattleADU / DADU
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About the author
Harrison Alley is a web publisher and data enthusiast based in Dallas, Texas. He built Urbarama’s remodeling calculators and maintains them.
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