How Much Does an ADU Cost in Washington in 2026?

Planning-level pricing guide by ADUWizard.com
Updated for 2026 budgeting

Washington quietly became one of the best ADU states in the country, and most homeowners still do not realize how much changed. HB 1337 stripped a lot of the old local roadblocks statewide. What it did not change is the thing that actually decides your budget: the type of unit, the size, the lot, the utilities, and whether you are building new or converting existing space.

So this is the real budgeting version, not a generic national average. This is the planning guide I would use to estimate a Washington ADU in 2026, whether you are in Seattle, on the Eastside, down in Tacoma, or over in Spokane.

Quick disclaimer: This is a budgeting guide, not a quote and not legal advice. Actual bids vary by city, lot conditions, slope, soil, utility routes, energy code, and finish level. Washington also charges sales tax on construction labor and materials, which pushes real bids higher than in most states. Use these ranges to budget and to compare bids apples-to-apples.

Washington ADU Cost in 2026 (Quick Answer)

For most Washington homeowners, a realistic all-in ADU budget runs from the low six figures for a clean conversion to the mid-to-high $400Ks (and up) for a detached unit in an expensive metro. Region matters more here than in almost any other state, because Seattle labor and site conditions are a different world from Spokane.

Typical all-in totals by region (2026)

Project scenario	Typical size	Seattle / King	Eastside (Bellevue/Redmond)	Tacoma / Pierce & Snohomish	Spokane / Eastern WA
Garage conversion (best value when feasible)	400–700 sf	$90k–$220k	$95k–$230k	$75k–$180k	$50k–$120k
Basement / interior ADU (egress + moisture risk)	450–800 sf	$170k–$330k	$180k–$340k	$130k–$270k	$95k–$210k
Attached ADU / addition (tie-in complexity)	500–900 sf	$220k–$400k	$240k–$420k	$175k–$330k	$130k–$260k
Detached new-build (the classic backyard DADU)	500–900 sf	$300k–$600k	$320k–$620k	$150k–$350k	$110k–$260k
Prefab / modular installed (predictable schedule)	500–900 sf	$260k–$480k	$270k–$500k	$170k–$340k	$135k–$280k

What these numbers assume

A typical 1–2 bedroom ADU, simple design, standard finish package
No extreme slope, no retaining walls, no major off-site utility work
Normal contingency included
Washington sales tax included (this alone can add tens of thousands in high-tax cities)

Washington ADU Law Status: HB 1337 (What It Means for Your Budget)

Here is the part that changed everything, and the part most builders still explain wrong. In 2023 Washington passed HB 1337, and by 2026 cities inside urban growth areas were required to bring their codes into compliance. It preempts a lot of local resistance.

The cost-relevant pieces:

HB 1337 rule	What it means	Why it matters for cost
Two ADUs per lot	Cities in UGAs must allow up to two ADUs per residential lot	You can plan a detached plus a conversion, or two detached, without a special process
No owner-occupancy	Cities cannot require you to live on-site (except as an STR condition)	Removes the biggest financing and resale drag on ADUs
Parking near transit	No off-street parking required within a half mile of major transit	Paving, drainage, and curb cuts are expensive to skip
Impact fees cut	Impact fees for ADUs capped at 50% of the single-family rate	Can save several thousand dollars per unit
No size floor below 1,000 sf	Cities must allow ADUs of at least 1,000 sf	Bigger by-right envelope, fewer variance fights

This is genuinely strong policy, on par with Massachusetts and California. The catch, and there is always a catch, is that cities still control the expensive stuff: setbacks, height, design review, tree rules, stormwater, and energy code. The state opened the door. Your lot and your city decide the price.

The Washington ADU Cost Formula (Fast Estimate)

For a quick planning number, I use the same approach everywhere in Washington:

Total ADU budget = (all-in $/sf) × (size), then stress-tested for site and tax.

The pieces that move it most:

Build type. Detached new construction is the top of the range; conversions are the value play.
Region. Seattle and the Eastside carry a labor and tax premium; Spokane and Eastern WA are meaningfully cheaper.
Sitework and utilities. Long sewer or electrical runs are the number-one wildcard.
Sales tax. Washington taxes construction, so a 9% to 10.25% rate quietly rides on top of everything.

Washington ADU Cost per Square Foot (2026)

If you want the simplest heuristic, use all-in dollars per square foot and then add sitework and tax risk on top.

Region	Conversion ADU ($/sf all-in)	Detached ADU ($/sf all-in)
Seattle / King County	$280–$480	$350–$650
Eastside (Bellevue / Redmond / Kirkland)	$300–$500	$380–$650
Tacoma / Pierce & Snohomish	$230–$400	$300–$520
Spokane / Eastern WA	$190–$340	$260–$450

How to use this: pick a reasonable number for your region, multiply by your planned size, then add sitework and Washington sales tax. Basic finishes sit near the bottom of each range, high-end near the top.

Washington ADU Cost by Type (2026)

A) Detached New-Build ADU (the classic backyard cottage)

The most predictable layout and the top of the budget. This is where Seattle and Spokane diverge hardest.

Size	Seattle / King	Tacoma / Pierce	Spokane / Eastern WA
500 sf	$290k–$480k	$150k–$300k	$110k–$230k
700 sf	$320k–$560k	$175k–$330k	$130k–$250k
900 sf (near the cap)	$360k–$600k+	$210k–$380k	$150k–$280k

When detached costs spike in Washington

long trenching to sewer, water, and electric
hillside lots, poor soils, and retaining walls
significant tree-retention requirements
stormwater and drainage review on tight urban lots

B) Garage Conversion ADU (best value when the structure cooperates)

Size	Seattle / King	Tacoma / Pierce	Spokane / Eastern WA
400 sf	$90k–$170k	$75k–$140k	$50k–$95k
600 sf	$120k–$210k	$100k–$175k	$70k–$120k

Conversion must-checks in Washington

slab moisture and insulation (real in the wet western half)
ceiling height and egress
panel and service upgrades (common with heat pumps)
fire separation from the primary home

C) Basement / Interior Conversion ADU

Great in Washington’s older housing stock, but this is where budgets get surprised.

Size	Seattle / King	Tacoma / Pierce	Spokane / Eastern WA
450 sf	$170k–$270k	$130k–$220k	$95k–$170k
700 sf	$210k–$330k	$165k–$270k	$130k–$210k

Why basements get expensive: egress window cutting, moisture control, headroom fixes, and reworking plumbing to keep proper slope.

D) Attached ADU (addition)

Size	Seattle / King	Tacoma / Pierce	Spokane / Eastern WA
600 sf	$220k–$350k	$175k–$290k	$130k–$225k
800 sf	$260k–$400k	$205k–$330k	$155k–$260k

Attached ADU risk point: tie-ins to the existing home (structure, roof, drainage, and MEP) can get complicated fast. Additions are either cheap per square foot or a headache, rarely in between.

E) Prefab / Modular Installed

Prefab often wins on schedule and predictability in Washington. It does not remove the foundation, utilities, or permit work.

Size	Seattle / King	Tacoma / Pierce	Spokane / Eastern WA
500 sf	$260k–$400k	$170k–$300k	$135k–$230k
800 sf	$300k–$480k	$215k–$340k	$170k–$280k

Prefab reality check: the factory price is one slice. Foundation, delivery, crane day, and utility hookups are often the same as stick-built.

Cost by Size and Bedrooms (Washington Planning Table)

Layout goal	Typical size	Conversion (Seattle)	Detached (Seattle)
Studio	350–500 sf	$90k–$220k	$260k–$430k
1 bedroom	500–700 sf	$120k–$300k	$300k–$520k
2 bedroom (near the cap)	700–1,000 sf	$170k–$360k	$340k–$600k+

Finish Level: What It Does to a Washington ADU Budget

Finishes move budgets more than most homeowners expect.

Finish level	Typical impact	Example on a $360k detached ADU
Value / builder grade	baseline	$360k
Mid-range (most common)	+5% to +15%	$378k–$414k
High-end / custom	+15% to +35%+	$414k–$486k+

Common upgrade adders

Upgrade	Typical adder	Notes
Ducted heat pump vs mini-splits	+$6k–$20k	Comfort and code driven
Second bathroom	+$12k–$30k	MEP, tile, waterproofing
Premium kitchen	+$8k–$40k+	Cabinetry scales fast
High-performance windows	+$5k–$25k+	Energy code plus labor
Covered deck / entry	+$5k–$35k+	Footings, rails, flashing

The Washington Hidden-Cost Checklist (Do Not Skip)

This is where Washington budgets move quickly.

Hidden cost item	When it happens	Planning range	How to reduce risk
Sales tax on construction	every project (9%–10.25%)	9%–10.25% of the taxable total	budget it from day one, it is not optional
Electrical panel / service upgrade	heat pumps, EVs, older panels	$3k–$15k+	load calc early
Long utility trenching	detached ADU far from the house	$5k–$30k+	site the ADU near existing utilities
Stormwater / drainage review	tight urban lots, new impervious area	$5k–$50k+	drainage plan early
Tree retention	significant trees on site	$2k–$20k+	survey trees before you design
Slope, soils, retaining walls	hillside lots (common around Puget Sound)	$15k–$80k+	geotech early, avoid slope if you can

Three Washington Example Budgets (All-In)

Example A: 550 sf garage conversion (1-bed), Seattle

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: 750 sf detached ADU (2-bed), Tacoma

Category	Budget
Design + engineering	$16,000–$32,000
Permits + fees	$6,000–$16,000
Sitework + utilities	$20,000–$50,000
Foundation + framing + envelope	$70,000–$130,000
MEP	$40,000–$70,000
Interior finishes	$30,000–$65,000
Sales tax + contingency	$22,000–$45,000
Total all-in	$175,000–$330,000

Example C: 900 sf detached ADU (high-end), Seattle / Eastside

Category	Budget
Design + engineering	$25,000–$55,000
Permits + fees	$10,000–$30,000
Sitework + utilities (moderate)	$35,000–$85,000
Foundation + envelope	$150,000–$250,000
MEP (ducted heat pump)	$70,000–$120,000
High-end finishes	$80,000–$160,000
Sales tax + contingency	$60,000–$110,000
Total all-in	$420,000–$650,000+

How to Lower Washington ADU Cost (Without Regret)

Cost lever	What to do	Why it saves
Site near utilities	Short sewer and electric runs	Utilities are the number-one wildcard
Keep the footprint simple	Rectangle plan, minimal bump-outs	Less foundation and framing complexity
Stack plumbing	Kitchen and bath share a wet wall	Cuts plumbing runs and venting
Convert instead of build	Garage or basement when viable	Skips foundation and most sitework
Design to your city’s fee triggers	Stay within by-right size and height	Avoids design review and variance costs
Lock finishes early	Set allowances before permit	Prevents change-order creep

Washington ADU Timeline (Planning)

Phase	Conversion	Detached new-build
Feasibility + concept	2–4 weeks	2–6 weeks
Design + engineering	4–8 weeks	6–12+ weeks
Permit review	4–12 weeks	8–16+ weeks
Construction	8–16 weeks	16–36+ weeks
Total, start to finish	6–11 months	10–18+ months

Seattle’s SDCI review typically runs 8 to 16 weeks; Spokane is often faster at roughly 50 to 75 days; smaller Eastside cities can clear plan review in 3 to 8 weeks. Design review, tricky sites, and shoreline or critical-area rules stretch all of it.

Main Washington Cities

Costs, rules, and permit timelines vary a lot city to city. Here is how Washington’s four biggest ADU markets compare, cheapest to priciest, before you drill into your metro:

City	Detached ADU (all-in)	Garage conversion	Permit review	Sales tax
Spokane	$110k–$260k	$50k–$120k	50–75 days	~9%
Tacoma	$150k–$350k	$75k–$180k	3–5 months	~10.3%
Seattle	$300k–$600k+	$90k–$220k	8–16 weeks	10.25%
Bellevue / Eastside	$320k–$620k+	$95k–$230k	3–8 weeks	~10.3%

Spokane is the clear value play; Seattle and the Eastside sit at the top of the market. Full breakdowns by city:

For the wider picture, compare our California ADU cost data and Oregon cost breakdown, and the Data Hub tracks ranges as they move. If financing is the real question, our guide to ADU financing options walks through how to pay for it.

Resources (Official Washington ADU Guidance)

Washington State Legislature, HB 1337 bill summary
MRSC (Municipal Research and Services Center of Washington), Accessory Dwelling Units guidance
Seattle SDCI for city permitting, and your local planning department for city-specific standards

FAQs (Washington)

How much does an ADU cost in Washington in 2026?

For most homeowners, budget roughly $90k–$220k for a garage conversion and $150k–$600k for a detached ADU, depending heavily on region. Seattle and the Eastside are the most expensive; Tacoma, and especially Spokane and Eastern Washington, run meaningfully cheaper.

Are ADUs legal statewide in Washington?

Effectively yes. HB 1337 requires cities in urban growth areas to allow up to two ADUs per lot, drop owner-occupancy, and ease parking and impact fees, and cities had to comply by 2026. Local standards for setbacks, height, design, and stormwater still apply.

Why are Washington ADUs more expensive than the national average?

Two big reasons: Washington taxes construction labor and materials (9% to 10.25%), and Puget Sound labor and site conditions (slope, soils, trees, stormwater) run well above the national norm. The tax alone can add tens of thousands.

What is the cheapest way to build an ADU in Washington?

Usually a garage or basement conversion, when the existing structure is sound and utility runs are short. In Spokane and Eastern Washington, even a small detached unit can pencil out well below Seattle prices.

Do I have to live on the property to build an ADU?

No. HB 1337 bars cities in urban growth areas from requiring owner-occupancy, though a city may still require it as a condition of running a short-term rental. Always confirm your specific city’s current code.

ADU Costs in Washington