From 574 standalone detached-ADU permits · updated October 14, 2026
What an ADU really costs in Seattle
Not estimates from contractors' marketing pages — these are the costs, permitting times, and construction times declared on actual Seattle building permits.
Detached ADU (backyard cottage) costs by year
Estimated project cost as declared on standalone DADU permits — projects that also build a new primary house are excluded, so these numbers reflect the cottage alone. Years with fewer than 10 permits are omitted rather than published as a median (2019 (9), 2021 (7)).
| Year issued | Permits | Median cost | Middle 50% range |
|---|---|---|---|
| 2020 | 11 | $104,565 | $79,043 – $150,000 |
| 2022 | 41 | $173,344 | $137,056 – $198,076 |
| 2023 | 48 | $182,710 | $152,751 – $214,476 |
| 2024 | 42 | $189,131 | $150,227 – $212,473 |
| 2025 | 17 | $191,075 | $184,674 – $213,662 |
| 2026 | 37 | $230,765 | $208,180 – $258,006 |
Permit-declared estimates typically run below final all-in cost (site work, finishes, and overruns land later) — treat these as a floor, not a quote. Attached-ADU permits are excluded from cost stats because most are filed with the main house's full construction cost.
Where Seattle is building ADUs
New-construction ADU permits by ZIP code since 2023.
| ZIP | Permits since 2023 |
|---|---|
| 98115 | 209 |
| 98117 | 205 |
| 98118 | 169 |
| 98125 | 148 |
| 98103 | 148 |
| 98108 | 124 |
| 98199 | 103 |
| 98133 | 102 |
| 98116 | 98 |
| 98106 | 92 |
Comparing builders? The rankings show who has actually completed ADU permits in city records, with license verification.