How much does adu construction cost in Seattle, WA?

Most Seattle homeowners spend between $192,000 – $384,000 on adu construction, with an average around $288,000. Local labor prices 28% above the national average, so Seattle quotes tend to land at the higher end of national ranges.

Why is adu construction more or less expensive in Seattle than elsewhere?

Seattle trade labor is among the most expensive in the country and books months ahead. Persistent rain demands drainage detailing, flashing and rot-resistant materials.

Do I need a permit for adu construction in Seattle, Washington?

SDCI permits and inspections are required for most exterior and structural work. Always confirm current requirements with the local building department before signing a contract, and make sure the permit is pulled by your licensed contractor.

How many quotes should I get in Seattle?

Get at least three written quotes from licensed, insured Washington contractors. Comparing line items — not just the bottom line — is the fastest way to spot an inflated bid in the Seattle market.

How much does adu construction cost in 2026?

Most homeowners pay between $150,000 and $300,000, with a national average near $225,000. Small, simple jobs land at the low end; large or premium-material jobs run well above the high end.

What is the cost per square foot?

Installed pricing usually runs $250.00 to $500.00 per square foot, including labor and materials. Expensive metros can run 15–45% higher than that range.

What drives the price up the most?

Scope and access. Larger quantities lower your per-square foot price, but difficult access, demolition of existing work, structural repairs found mid-job, premium materials and rush scheduling each add 8–30%.

Permits are typically required when the work changes structure or ties into electrical, plumbing, gas or mechanical systems. Budget $75–$600 in most cities and confirm whether your contractor pulls it — unpermitted work can block a future home sale.

How long does the job take?

Most residential projects of this type finish in one to five working days once materials are on site. Custom orders, inspections and weather are the usual causes of delay.

How many quotes should I get?

Three. Bids on the same scope commonly vary 20–40%, and the middle bid with the strongest warranty and references is usually the best value — the cheapest bid often excludes prep, disposal or permits.

Home/Cost guides/ADU Construction/Seattle, WA

Seattle · Washington

ADU Construction cost in Seattle, WA

Seattle homeowners typically pay $192,000 – $384,000 for adu construction. Local labor and materials price 28% above the national average.

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Seattle average

$288,000

Typical low

$192,000

Typical high

$384,000

Per square foot

$320 – $640

Sample ZIP

98101

What drives Seattle pricing

Local labor

Seattle trade labor is among the most expensive in the country and books months ahead.

Permits & code

SDCI permits and inspections are required for most exterior and structural work.

Climate & conditions

Persistent rain demands drainage detailing, flashing and rot-resistant materials.

Instant cost calculator

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ADU size (sq ft)600 square feet

Quality / material level

Site access

Removal / prep work

Timing

Estimated project cost

$171,000 – $233,700

Most homeowners pay around $202,350

Compare quality / material level

Cost by material in Seattle

Installed Seattle pricing per square foot.

Option	Installed cost	Pros	Cons
Prefab studio ADU	$300 – $410	—	—
Site-built 1-bedroom	$410 – $540	—	—
Site-built 2-bedroom	$540 – $695	—	—
Custom high-end ADU	$695 – $960	—	—

Where the money goes in Seattle

A typical $288,000 Seattle project, broken down by line item.

Labor0.45% · $1,296
Materials0.38% · $1,094
Permits & inspections0.05% · $144
Overhead & profit0.12% · $346

Common add-ons

Extras that show up on Seattle quotes but rarely in the headline price.

Utility connections

$10,200 – $38,400

Site work and foundation

$19,200 – $76,800

Design and permits

$10,200 – $38,400

Impact fees

$2,550 – $25,600

Ways to save in Seattle

Use pre-approved ADU plans where your city offers them — saves months and design fees.
Detach only if required; attached and conversion ADUs cost far less.
Collect three written bids on an identical scope — spreads of 20–40% are normal.
Book in the off-season and stay flexible on the start date to save 5–15%.
Bundle nearby work into one mobilization so you only pay setup and travel once.
Ask what the contractor would change to cut 10% without hurting durability.

Seattle FAQs

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More Seattle research

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ADU Construction cost near Seattle

ADU Construction cost in Bellevue, WA

$198,000 – $396,000

ADU Construction cost in Tacoma, WA

$172,500 – $345,000

ADU Construction cost in Everett, WA

$177,000 – $354,000

ADU Construction cost in Kent, WA

$178,500 – $357,000

ADU Construction cost in Renton, WA

$180,000 – $360,000

More Washington cities

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Other project costs in Seattle

Fence Installation in Seattle

$1,920 – $5,120

Roof Replacement in Seattle

$10,880 – $43,520

Kitchen Remodel in Seattle

$23,040 – $121,600

HVAC Replacement in Seattle

$7,040 – $23,040

Deck Building in Seattle

$5,632 – $15,360

Window Replacement in Seattle

$576 – $1,920

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