Field Notes · July 30, 2026 · 6 min read
What Does an ADU Cost in Snohomish & King County?
Backyard ADU and DADU budgets for the Seattle area in 2026 — what drives cost, how Washington's new rules help, and how to check what your lot allows.
ADUs went from novelty to mainstream in the Seattle area almost overnight — helped by state housing laws that forced cities to make them easier to approve. Here’s what they actually cost to build, and how to find out what your lot allows.
The three flavors, roughly priced
- Garage conversion (often $100k–$150k+). The structure exists; the money goes to insulation, systems, a kitchen and bath, and bringing it to habitable code.
- Attached ADU (often $150k–$250k). A basement or addition converted to a separate dwelling — cost tracks how much new plumbing and access work is needed.
- Detached ADU / DADU (often $250k–$400k+). A ground-up small home: foundation, utilities, full envelope. Site conditions — slope, trees, utility distance — swing this more than finishes do.
What the new rules changed
Washington’s recent legislation made cities across Snohomish and King County allow ADUs more broadly — commonly two per lot now — with softened parking and owner-occupancy requirements. The catch: each city still sets its own size, height and setback limits, which is why we start every ADU conversation with a feasibility check on your specific parcel. Our permits guide explains how the approval process runs.
Budget honestly, build once
Utility connections are the classic surprise — trenching water, sewer and power across a yard isn’t glamorous, but it’s real money. So is designing for rental durability if that’s the goal. We build ADUs as part of our additions & ADU service, permitted and warrantied, with one owner accountable end to end.
Frequently asked questions
- How much does a backyard ADU cost near Seattle?
- As broad 2026 ranges: garage conversions often start around $100k–$150k; new detached ADUs (DADUs) commonly run $250k–$400k+ depending on size, site work and finishes. Utility connections and sloped lots are the big wildcards — get a lot-specific estimate.
- Are ADUs legal in Snohomish and King County?
- Yes — Washington's recent housing laws pushed cities to allow ADUs broadly, and many now permit two per lot with relaxed parking and owner-occupancy rules. Exact size and height limits vary by city, so a feasibility check on your parcel is step one.
- Does an ADU add value to my home?
- Appraisers increasingly count permitted, legal ADU square footage, and rental demand in the Seattle area is strong. Unpermitted units, by contrast, hurt at resale — permits are non-negotiable.
Written by
Eduard — Owner & General Contractor
Eduard owns ENF General Contracting and has built and remodeled homes across Everett, Seattle & the Eastside for over a decade. Licensed, Bonded & Insured. He runs every ENF project personally — and writes these guides from the job site. More about Eduard · (425) 217-9921