An ADU builder in Seattle is a licensed contractor who designs and constructs accessory dwelling units — detached backyard units, garage conversions, attached additions, and junior ADUs (JADUs). Because an ADU is a complete second dwelling, the builder coordinates foundation, framing, plumbing, electrical, and HVAC work and pulls it through Seattle's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in Washington.
What ADU Builders Cost in Seattle [4][5]
Based on our Seattle cost data, expect the following ranges from local ADU builders. Quotes for the same scope typically vary 20-40% between contractors, so getting at least three bids is the single best way to control cost.
| New Detached ADU (all-in) | $160,000-$320,000 |
| Cost per Sq Ft | $130-$270 |
| Garage Conversion | $75,000-$170,000 |
| Prefab / Modular ADU (installed) | $110,000-$260,000 |
| City Permit & Plan Review | $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) |
| Impact Fees | ADUs exempt from MHA affordability contributions and most impact fees |
For a full line-item breakdown, see our Seattle ADU cost guide. To confirm what you're allowed to build before you hire, review the Seattle ADU rules (max detached size: 1,000 sq ft of floor area above grade (size also constrained by FAR and lot coverage); owner-occupancy: Not required).
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How to Vet an ADU Builder in Seattle
- License: Confirm a current contractor's license in Washington and that it's in good standing. Ask for the license number and verify it with the state board.
- ADU-specific experience: Ask how many ADUs they've completed in or near Seattle and request addresses or photos. ADU permitting and setbacks differ from a kitchen remodel.
- Portfolio & references: Get references from recent ADU clients and, if possible, visit a finished unit.
- Insurance: Verify general liability and workers' compensation coverage. Ask for a certificate of insurance.
- Written, detailed bid: Insist on an itemized, fixed-price or clearly-scoped bid — not a verbal estimate. It should state whether permits ($2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply)), plan check (1-3 weeks for pre-approved (ADUniverse) plans; longer for custom), and impact fees (ADUs exempt from MHA affordability contributions and most impact fees) are included.
- Permit handling: Confirm they manage Seattle's permit process ("Building permit (no Master Use Permit required for most ADUs); SDCI review", typically Several weeks to a few months depending on review queue) and who is responsible if revisions are required.
- Payment schedule: Avoid large upfront payments. A reasonable schedule ties payments to completed milestones.
Questions to Ask Before You Hire
- How many ADUs have you built in Seattle in the last two years?
- Is your bid fixed-price, and what would trigger a change order?
- Are permits, plan check, and utility connection fees included?
- What's a realistic timeline, including permit review (Several weeks to a few months depending on review queue) and construction?
- Who is my point of contact and how are delays communicated?
- What warranty do you provide on workmanship?
- Can you share three recent Washington ADU client references?
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