Planning-level pricing guide by ADUWizard.com
Updated for 2026 budgeting
Seattle has been building backyard cottages longer than almost any West Coast city, and it shows: the rules are workable, the demand is real, and the prices are steep. A detached ADU here is not a cheap project. It is a small custom home, priced like one.
This is the Seattle version of the real budgeting guide. Not national averages, not a builder’s teaser number. This is what I would actually plan around for a DADU, garage conversion, or basement unit in Seattle in 2026, including the SDCI rules that shape what you can build and the site conditions that decide the price.
Quick disclaimer: This is a budgeting guide, not a quote or legal advice. Seattle bids swing hard on slope, soil, tree rules, and utility runs, and Washington sales tax (10.25% in Seattle) rides on top of everything. Use these ranges to compare bids, not as a fixed price.
Seattle ADU Cost in 2026 (Quick Answer)
| Project scenario | Typical size | Seattle all-in cost |
|---|---|---|
| Garage conversion | 400–650 sf | $90k–$220k |
| Basement / interior ADU | 450–800 sf | $170k–$330k |
| Attached ADU / addition | 500–900 sf | $220k–$400k |
| Detached new-build (DADU) | 500–1,000 sf | $300k–$600k+ |
| Prefab / modular installed | 500–900 sf | $260k–$480k |
Most homeowners building a 600–800 sf DADU should plan $400k–$600k all-in, including permits and fees. Seattle’s all-in cost per square foot runs roughly $350–$500 for a standard build and can top $650 on hard sites.
Seattle ADU Rules That Affect Cost (2026)
Seattle already met, and in places exceeds, Washington’s statewide HB 1337 baseline. It dropped owner-occupancy and parking back in 2019, well before the state required it. Here is what the current SDCI code actually allows, and why each piece matters for your budget.
| Rule | Seattle standard (2026) | Why it matters for cost |
|---|---|---|
| ADUs per lot | Up to two (one attached plus one detached) | You can plan a DADU and a basement unit on one lot |
| Max DADU size | 1,000 sf, or 60% of the primary home, whichever is greater | A real 1–2 bedroom unit is by-right |
| Max height | 24 ft (pitched roof) or 18 ft (flat) | Going taller can trigger larger setbacks |
| Setbacks | 5 ft side and rear; DADU behind the primary or 20 ft from the front line | Tight lots may force a smaller footprint |
| Parking | None required (removed in 2019) | Saves paving, drainage, and curb-cut cost |
| Owner-occupancy | Not required (removed in 2019) | Removes the biggest financing and resale drag |
What still drives cost: tree-retention rules, stormwater and drainage review, and slope and soils on Seattle’s many hilly lots. The permission is easy. The site is where the money goes.
Seattle ADU Cost by Type (2026)
How much does a garage conversion cost in Seattle?
A garage conversion is the value play, generally $90k–$220k all-in when the structure and slab cooperate. The budget hinges on slab moisture, ceiling height, egress, and whether the panel and service need an upgrade for a heat pump.
How much does a basement ADU cost in Seattle?
Most basement conversions run $170k–$330k. Egress-window cutting, moisture control, and headroom fixes are the usual surprises in Seattle’s older housing stock.
How much does a detached DADU cost in Seattle?
A detached backyard cottage is the top of the range at $300k–$600k+. It carries its own foundation, utilities, and full envelope, so it prices like the small custom home it is.
| Type | Typical size | All-in range | Notes |
|---|---|---|---|
| Garage conversion | 400–650 sf | $90k–$220k | Best value if the structure and slab cooperate |
| Basement / interior | 450–800 sf | $170k–$330k | Egress, moisture, and headroom drive cost |
| Attached addition | 500–900 sf | $220k–$400k | Tie-ins to the existing home add complexity |
| Detached DADU | 500–1,000 sf | $300k–$600k+ | A small custom home, priced like one |
Seattle ADU Cost by Size
| Size | Garage / basement conversion | Detached DADU |
|---|---|---|
| 400–500 sf (studio) | $90k–$200k | $290k–$430k |
| 600–700 sf (1-bed) | $150k–$300k | $320k–$520k |
| 800–1,000 sf (2-bed) | $200k–$360k | $360k–$600k+ |
Seattle cost per square foot
| Finish level | All-in $/sf |
|---|---|
| Basic / builder grade | $220–$350 |
| Mid-range (most common) | $400–$500 |
| High-end / custom | $500–$650+ |
The Seattle Cost Drivers Nobody Budgets For
| Hidden cost | When it hits | Planning range |
|---|---|---|
| Sales tax (10.25%) | every project | 10.25% of the taxable total |
| Soft costs (design, permits, fees) | every project | $20k–$50k |
| Hilly / difficult site (slope, soils, access) | common in Seattle | $15k–$60k+ |
| Tree retention | protected trees on site | $2k–$20k+ |
| Utility trenching | DADU far from the house | $10k–$30k |
Seattle’s skilled-trade labor runs roughly 20% to 30% above the statewide average, and that flows straight into every line item.
Two Seattle Example Budgets (All-In)
Example A: 550 sf garage conversion (1-bed)
| Category | Budget |
|---|---|
| Design + engineering | $12,000–$22,000 |
| Permits + fees (SDCI) | $8,000–$25,000 |
| Demo + prep | $6,000–$18,000 |
| MEP (heat pump, plumbing, electrical) | $35,000–$70,000 |
| Insulation, drywall, finishes | $45,000–$95,000 |
| Sales tax + contingency | $18,000–$35,000 |
| Total all-in | $120,000–$220,000 |
Example B: 700 sf detached DADU (2-bed)
| Category | Budget |
|---|---|
| Design + engineering | $18,000–$35,000 |
| Permits + fees (SDCI) | $16,000–$40,000 |
| Sitework + utilities | $30,000–$70,000 |
| Foundation + framing + envelope | $110,000–$190,000 |
| MEP | $55,000–$95,000 |
| Interior finishes | $45,000–$95,000 |
| Sales tax + contingency | $45,000–$90,000 |
| Total all-in | $400,000–$600,000+ |
Seattle ADU Permit Timeline
| Phase | Typical duration |
|---|---|
| Design + engineering | 6–12 weeks |
| SDCI permit review | 8–16 weeks |
| Construction | 16–32 weeks |
| Total, start to finish | 9–14 months |
Basement and garage conversions can move faster than a ground-up DADU, but SDCI review is the pacing item either way.
FAQs (Seattle)
How much does a DADU cost in Seattle in 2026?
Most detached backyard cottages run $300k–$600k or more all-in, and a typical 600–800 sf DADU lands around $400k–$600k including permits and fees. Sales tax and site conditions are the biggest swing factors.
What is the cheapest ADU to build in Seattle?
A garage conversion, usually $90k–$220k when the existing structure is sound and utility runs are short. A basement conversion is the next-cheapest path in Seattle’s older housing stock.
How big can a DADU be in Seattle?
Up to 1,000 sf, or 60% of the primary home’s floor area, whichever is greater, with a height cap of 24 feet for a pitched roof (18 feet flat). That supports a genuine 1–2 bedroom unit on most lots.
Do I need parking or to live on-site for a Seattle ADU?
No to both. Seattle removed the off-street parking requirement and the owner-occupancy requirement in 2019, so you can build and rent without living on the property.
How long does a Seattle ADU take?
Plan on 9–14 months start to finish, with SDCI permit review alone running 8–16 weeks. Hard sites, tree rules, and design choices extend it.
For the statewide picture, see our Washington ADU cost guide, compare cheaper Tacoma and Spokane, and the Data Hub tracks ranges as they move. City permitting details live at Seattle SDCI.