Washington ADU Cost in 2026
Building an ADU in Washington typically costs between $180,000 and $420,000 in 2026, depending on size, location within the state, and finish level.
Typical range
$180k–$420k
Per sqft
$250–$500
Where the money goes
Line-item averages for a typical ADU build. Actual percentages vary by site conditions and finish level.
Foundation
$25k–$45kFraming & shell
$60k–$120kMechanical / electrical / plumbing
$35k–$70kFinishes
$30k–$90kPermits & impact fees
$10k–$30kDesign & engineering
$15k–$35kWashington ADU costs are influenced by several state-specific factors: local labor rates, permit fees, soft costs from design and engineering, and impact fees from utilities and schools.
Within Washington, costs vary significantly by metro area. Major urban markets typically run 20-40% higher than rural or suburban projects.
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