A garage conversion is usually the lowest-cost path to an ADU in Seattle — $75,000-$170,000, versus $160,000-$320,000 for a new detached build — because the shell already exists.
Garage Conversion Cost in Seattle
Converting an existing garage in Seattle runs $75,000-$170,000. That's well under the $160,000-$320,000 typical cost of a ground-up detached ADU, because you're reusing the foundation, walls, and roof. The biggest variables are whether the slab and framing are sound, what utility/electrical upgrades are needed, and your finish level. Permit fees add $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply). A converted unit rents for about $1,600-$2,100/mo (1BR) in Seattle.
The Seattle Garage Conversion Permit Process
A garage-to-ADU conversion in Seattle needs building permits even if the exterior is unchanged. Approval process: Building permit (no Master Use Permit required for most ADUs); SDCI review The review timeline is Several weeks to a few months depending on review queue, with plan check: 1-3 weeks for pre-approved (ADUniverse) plans; longer for custom. Pre-approved plans: Yes — ADUniverse pre-approved DADU plan library.
- Confirm eligibility — ADUs are allowed in All residential zones (Neighborhood Residential and other residential zones).
- Prepare conversion plans — layout, egress windows, insulation, ventilation, smoke/CO detectors, plus a kitchen and bathroom.
- Submit for plan check — 1-3 weeks for pre-approved (ADUniverse) plans; longer for custom.
- Pull permits and build — fees run $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply); then framing, electrical, plumbing, and final inspection.
Parking Rules for a Garage Conversion
Not required (no parking minimum for ADUs) Garage conversions sometimes trigger different parking rules than new detached ADUs, so confirm replacement-parking requirements with Seattle before you start. Owner-occupancy is not required, so you can rent the converted unit out.
The JADU Option for an Attached Garage
If your garage is attached to the house, you can convert it into a junior ADU (JADU) — capped at Seattle does not use a separate JADU category; second unit treated as an AADU/DADU and built within the existing walls. JADUs are cheaper and faster to permit than a full ADU, though they may share sanitation with the main home. A full ADU gives you more size and a separate unit; a JADU is the budget route.
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Seattle Garage Conversion FAQ
How much does a garage conversion cost in Seattle?
Do I need a permit to convert a garage in Seattle?
Do I have to replace the parking if I convert my garage in Seattle?
How long does a garage conversion take in Seattle?
Can I convert an attached garage into a JADU in Seattle?
Compare your options: see the full Seattle ADU rules, estimate your build with the Seattle ADU cost calculator, or read the general garage conversion guide.