Garage Conversion in Seattle, WA: Cost, Permits & Rules (2026)

A garage conversion is usually the lowest-cost path to an ADU in Seattle — $100,000-$225,000, versus $325,000-$550,000 for a new detached build — because the shell already exists.

Garage Conversion Cost in Seattle

Converting an existing garage in Seattle runs $100,000-$225,000. That's well under the $325,000-$550,000 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 $3,453 in plan-review and building-permit fees for a 500 sq ft DADU valued at $93,145, per SDCI's own 2026 fee example (up 18.7 percent from $2,908 in 2025). Fees are valuation-based (Fee Subtitle Table D-1); at SDCI's 2026 rate of $186.29 per sq ft for wood-frame R-3 construction, the 2026 Fee Estimator gives roughly $4,990 for a 1,000 sq ft DADU, 75 percent due at intake. Add a 5 percent technology fee, the $6.50 state surcharge, hourly reviews at the 2026 base rate of $292 per hour where triggered, plus separate electrical, plumbing and side sewer permits. A subject-to-field-inspection permit pays 40 percent of the plan-review fee, and a pre-approved standard-plan submittal gets a reduced plan-review fee (Tip 310). A converted unit rents for about $1,900-$2,600/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: Administrative SDCI construction permit, no Master Use Permit, no public comment period and no appeal (SDCI Tip 116B). In order: 1) research the parcel on the Seattle Services Portal for zone, ECA and shoreline flags; 2) file a Building & Land Use Pre-Application with a site plan, which returns a record number and any pre-application site visit; 3) book an intake appointment and upload PDF plans (site plan, floor plans for house and ADU, elevations, structural, energy forms); 4) pay 75 percent of plan-review and permit fees at intake; 5) SDCI zoning, ordinance/structural and other reviews with correction cycles; 6) pay the balance, permit issued, inspections, final inspection before a tenant moves in. A new DADU or a unit inside the house is a Construction Addition/Alteration permit; legalizing an existing unit is a Construction Permit to Establish Use. A separate SDCI electrical permit, a King County Public Health plumbing permit and usually a side sewer permit are needed, and SDCI reports every ADU permit to King County for the sewer capacity charge The review timeline is No published SDCI target for ADU review. Pre-approved ADUniverse plans: 2-6 weeks to permit on most sites (SDCI and OPCD). Custom plans: SDCI's Tip 116B warns the intake appointment alone may be 2-4 months out, and 2026 industry guides report 4-8 weeks of review for a straightforward custom DADU and 12-18 months from design start to move-in overall (8-10 months with a pre-approved plan). SDCI's Construction Permit Timeline tool (April 2026) shows the actual review history of any permit since 2018, with plan check: Electronic plan review through the Seattle Services Portal. Plans must state the number of ADUs, whether the unit is in a new or existing structure, the ADU size, total lot coverage and any parking provided (Tip 116B). The ADU must meet the Seattle Residential, Building, Energy, Stormwater and side sewer codes; a converted structure must fully meet the Energy Code; sleeping rooms need egress windows (5.7 sq ft opening, sill no higher than 44 in.); the ADU needs its own heating controls plus smoke and CO alarms. Tips 116A and 116B were last updated Aug. 18, 2023 and still print the pre-2025 rules (3,200 sq ft lot, 40 percent rear-yard rule, 650 sq ft in Lowrise), which the 2025-26 ordinances superseded. Pre-approved plans: Yes. The City's ADUniverse gallery lists 7 pre-approved DADU designs (10 floor plans, 288 to 1,000 sq ft, studio to 3 bedrooms). Using one gets a permit in 2-6 weeks on most sites and a reduced plan-review fee, but only on a Neighborhood Residential lot with no environmentally critical area and under 750 sq ft of ground disturbance (SDCI help article, May 2026). ADUniverse 2.0 selected 15 new ADU and middle-housing designs from 110 submissions in 2026; the City expects them to be available from January 2027.

1. Confirm eligibility — ADUs are allowed in Every zone where housing uses are allowed (SMC 23.42.022.A): Neighborhood Residential NR1-NR3, Lowrise LR1-LR3, Midrise, Highrise, Neighborhood Commercial, Seattle Mixed and downtown zones. In the Shoreline District an ADU must also meet Chapter 23.60A. ADUs may only be accessory to a housing use, and both units must be on the same lot (23.42.022.B and F).
2. Prepare conversion plans — layout, egress windows, insulation, ventilation, smoke/CO detectors, plus a kitchen and bathroom.
3. Submit for plan check — Electronic plan review through the Seattle Services Portal. Plans must state the number of ADUs, whether the unit is in a new or existing structure, the ADU size, total lot coverage and any parking provided (Tip 116B). The ADU must meet the Seattle Residential, Building, Energy, Stormwater and side sewer codes; a converted structure must fully meet the Energy Code; sleeping rooms need egress windows (5.7 sq ft opening, sill no higher than 44 in.); the ADU needs its own heating controls plus smoke and CO alarms. Tips 116A and 116B were last updated Aug. 18, 2023 and still print the pre-2025 rules (3,200 sq ft lot, 40 percent rear-yard rule, 650 sq ft in Lowrise), which the 2025-26 ordinances superseded.
4. Pull permits and build — fees run $3,453 in plan-review and building-permit fees for a 500 sq ft DADU valued at $93,145, per SDCI's own 2026 fee example (up 18.7 percent from $2,908 in 2025). Fees are valuation-based (Fee Subtitle Table D-1); at SDCI's 2026 rate of $186.29 per sq ft for wood-frame R-3 construction, the 2026 Fee Estimator gives roughly $4,990 for a 1,000 sq ft DADU, 75 percent due at intake. Add a 5 percent technology fee, the $6.50 state surcharge, hourly reviews at the 2026 base rate of $292 per hour where triggered, plus separate electrical, plumbing and side sewer permits. A subject-to-field-inspection permit pays 40 percent of the plan-review fee, and a pre-approved standard-plan submittal gets a reduced plan-review fee (Tip 310); then framing, electrical, plumbing, and final inspection.

Parking Rules for a Garage Conversion

None required for any ADU (SMC 23.42.022.I and Table B for 23.54.015, footnote 4). If you do build a space, it must be reached from an improved street, alley or easement, may not sit within 20 ft of the front lot line except on the driveway, and alley access is required where an improved alley exists (23.44.160). The state floor is the same: RCW 36.70A.681(2) bars any parking requirement for ADUs within a half mile of a major transit stop 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 has no junior ADU category. A second unit carved out of the house is simply a second attached ADU and counts toward the two-ADU limit 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.

Seattle Garage Conversion FAQ