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ADU / DADU Cost in Seattle, Washington
The median declared cost of a adu / dadu in Seattle, Washington is $183,517, based on 437 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 | $183,517 |
|---|---|
| Typical range (middle half) | $93,678 – $211,607 |
| Full spread (10th–90th percentile) | $34,204 – $250,000 |
| Permits in sample | 437 |
| 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 adu / dadu is filed together with other work, the single declared figure covers all of it — one reason the top of the range runs high.
Planning a backyard cottage of your own? A detached ADU stands on its own foundation — the concrete slab calculator works out the pour in yards and bags — and its budget scales with floor area, which the square footage calculator measures from your plans in a minute.
How the costs spread
Half of all permits in the sample declared between $93,678 and $211,607. The full spread is wider: the cheapest 10% declared under $34,204, and the most expensive 10% declared over $250,000.
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 | 47 | $173,162 |
| 98115 | 35 | $198,624 |
| 98103 | 33 | $103,791 |
| 98133 | 31 | $204,565 |
| 98126 | 30 | $186,548 |
How this project gets permitted in Seattle
Seattle permit descriptions name the review path each job actually took. Of the 378 permits in this sample whose descriptions name it, 99% went through full plan review (3 STFI vs 375 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 $183,517, the building permit and plan review on the full plan review path come to about $5,199, including the 5% technology fee and the $6.50 state building-code fee. Across the middle half of declared values ($93,678–$211,607), the same fees run about $3,646–$5,654. Trade permits (electrical, plumbing, mechanical) are charged separately.
Trend
Median declared cost was $192,737 in 2024 (39 permits) and $187,780 in 2026 (113 permits) — about flat across the two years.
| Year issued | Permits | Median declared cost |
|---|---|---|
| 2024 | 39 | $192,737 |
| 2025 | 285 | $173,950 |
| 2026 | 113 | $187,780 |
Recent adu / dadu 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 | Allow new detached accessory dwelling unit to existing single family use per land use code. Construct substantial alterations to create wes… | $76,789 |
| Nov 2026 | Allow new detached accessory dwelling unit to existing single family use per land use code. Construct new one family dwelling per plan (sta… | $252,237 |
| Nov 2026 | Allow new detached accessory dwelling unit accessory to existing residential use per land use code. Construct new one family dwelling per s… | $86,997 |
| Oct 2026 | Construct NW one-family dwelling (DADU) per plan. (Establish use as housing for four detached dwelling units per land use code. Existing de… | $347,642 |
| Oct 2026 | Construct new one family dwelling (West DADU) per plan. (Allow new detached accessory dwelling units to existing single family use per land… | $195,605 |
| Oct 2026 | Construct new one family dwelling (East DADU) per plan. (Allow new detached accessory dwelling units to existing single family use per land… | $186,290 |
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 adu / dadu cost in Seattle, Washington?
Based on 437 Seattle, Washington building permits issued in the last 24 months that included a adu / dadu, the median declared cost was $183,517. The middle half of permits declared between $93,678 and $211,607.
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 adu / dadu 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 adu / dadu in Seattle need plan review?
Most take the full plan review path: 99% of the 378 permits in our sample whose descriptions name the review path went that way (3 STFI, 375 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 adu / dadu in Seattle?
SDCI charges by declared value. At this project's median declared value of $183,517, the building permit plus plan review on the full plan review path comes to about $5,199, including the 5% technology fee and the $6.50 state fee (Seattle Municipal Code 22.900D.010). Trade permits are extra.
What is the difference between an AADU and a DADU?
An AADU (attached accessory dwelling unit) is created inside or attached to the existing house — a basement or garage conversion, or an addition. A DADU (detached accessory dwelling unit) is a separate structure, usually a backyard cottage. Seattle permits both under the same rules, and this sample combines them: the permit descriptions name the unit type, and both are filed as accessory dwelling units accessory to an existing home.
Plan the work
Concrete Slab Calculator
Cubic yards and bag counts for slabs and pads, with ready-mix vs bag cost comparison.
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Area for rooms and yards in any shape — rectangles, circles, triangles, L-shapes — with conversions and a project total.
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Shingle squares and bundles from footprint and pitch, with underlayment and ridge cap.
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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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