ADU Costs in Seattle, Washington

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.