ADU and backyard cottage cost in Seattle (2026):
what it costs and the City's rules after the 2025 changes
HOW MUCH DOES AN ADU COST IN SEATTLE?
Published figures vary widely. HomeGuide's national figures are $100 to $300 per square foot, or $60,000 to $180,000 for a 600 sq ft ADU. A Seattle builder, New Image, puts detached ADUs in Seattle at $350 to $500 per square foot, or about $300,000 to over $450,000 for a backyard cottage. Since June 30, 2025, Seattle allows up to two ADUs on a lot, generally up to 1,000 sq ft above ground each.
ADU cost: national and Seattle figures
| Source | Per square foot | Example |
|---|---|---|
| HomeGuide (national) | $100–$300 | 600 sq ft: $60,000–$180,000 |
| New Image, Seattle builder: detached ADU | $350–$500 | 600 sq ft: $210,000–$300,000 |
| New Image: detached backyard cottage | — | about $300,000 to over $450,000 |
| New Image: two-story DADU | — | $400,000–$600,000 |
Cost by ADU type
| Type | HomeGuide, per sq ft | HomeGuide, 600 sq ft | New Image, Seattle |
|---|---|---|---|
| Basement conversion | $50–$100+ | $30,000–$60,000 | $200,000–$300,000 |
| Garage conversion | $50–$200 | $20,000–$140,000 total | $180,000–$250,000 |
| Attached ADU | $125–$225 | $75,000–$135,000 | — |
| Detached ADU | $150–$250 | $90,000–$150,000 | $300,000–$450,000+ |
| Above a garage | $200–$500+ | $120,000–$300,000 | — |
Extra costs to plan for
| Item | Reported cost |
|---|---|
| Plans (drafting, engineer or architect input) | $1,200–$5,000 (HomeGuide) |
| Land survey | $400–$1,800 (HomeGuide) |
| Excavation | $1,500–$5,000 (HomeGuide) |
| Concrete slab foundation | $2,400–$10,500 (HomeGuide) |
| Seattle permits | $8,000–$15,000 (New Image) |
| Utility connections and impact fees | Varies; New Image says "several thousand dollars" |
- Sales tax. Washington taxes construction for a homeowner on the whole contract, labor included. On a $300,000 contract, Seattle's 10.55% rate adds about $31,650 (our calculation). See sales tax on home repairs.
- Impact fees are capped. State law (RCW 36.70A.681) says impact fees on an ADU can't be more than 50% of those on the principal home.
- Property tax relief. An ADU can qualify for Washington's three-year exemption on the added value if you file before it's finished: see the home improvement property tax exemption.
Seattle ADU rules after June 2025
| Rule | Seattle now |
|---|---|
| How many | Up to two ADUs on a lot with a principal home (attached, detached or one of each) |
| Size | 1,000 sq ft above ground in neighborhood residential (NR) and RSL zones; more allowed when converting space in an existing house |
| Height | Up to 32 ft in NR and RSL zones (was 18 ft) |
| Parking | No off-street parking required (since 2019) |
| Owner living on site | Can't be required under state law |
| Selling separately | Allowed as a condominium unit under state law |
- How many get built. Seattle permitted 913 ADUs in 2024: 448 attached and 465 detached (OPCD 2024 ADU Annual Report).
- Many are sold. OPCD says 57% to 58% of ADUs permitted in 2022 and 2023 were built on condo parcels, which suggests many are built to sell.
Permits and pre-approved plans
- Pre-approved DADU plans. Seattle's ADUniverse gallery offers pre-approved backyard cottage designs. OPCD says that in most cases you can get your permit in 2 to 6 weeks with one.
- SDCI tip sheets: Tip 116A covers attached ADUs and Tip 116B backyard cottages.
- Check the contractor. Look it up on L&I's Verify a Contractor tool, keep the first payment small (contractor down payments) and ask for lien releases (construction liens).
Frequently asked questions
How much does an ADU cost in Seattle?
Figures vary widely. HomeGuide's national range is $100 to $300 per square foot, while a Seattle builder, New Image, puts detached ADUs in Seattle at $350 to $500 per square foot, or about $300,000 to over $450,000 for a backyard cottage. Sales tax of 10.55% applies to the contract.
How many ADUs can I build in Seattle?
Up to two on a lot with a principal home, attached or detached, under rules that took effect June 30, 2025.
How big can an ADU be in Seattle?
1,000 square feet above ground in neighborhood residential and RSL zones, with more allowed when converting space in an existing house. Height can reach 32 feet in those zones.
Do I have to live on the property to build an ADU in Seattle?
No. Washington law (RCW 36.70A.681) bars cities from requiring the owner to live on a lot with an ADU.
How long does an ADU permit take in Seattle?
With one of Seattle's pre-approved backyard cottage plans, the City says you can get a permit in 2 to 6 weeks in most cases. Custom designs usually take longer.
About these prices. The ranges are published estimates from the sources below, as of the dates shown. Material costs, labor rates, tariffs and inflation can change prices after those dates, and your home may differ. Use them to plan a budget, then get written quotes from licensed contractors.
Sources
- Seattle Office of Planning and Community Development — Encouraging Backyard Cottages
- Seattle OPCD — 2024 ADU Annual Report (published November 2025)
- The Urbanist — Seattle eases accessory dwelling restrictions just ahead of state deadline (July 1, 2025)
- RCW 36.70A.681 — Accessory dwelling units
- HomeGuide — How much does it cost to build an ADU?
- New Image Construction Management — Seattle DADU cost (October 2025)
- Washington Department of Revenue — Custom construction
All sources accessed November 10, 2026.
This guide summarizes published cost estimates and City of Seattle and Washington rules. Zoning details are changing as Seattle adopts permanent middle housing rules. Confirm what you can build with SDCI before you design, and get written quotes from licensed, bonded contractors. See our disclosure and disclaimer.