SPEC_C / SEATTLE · GARAGE CONVERSION · Verified 2026-01-15
Seattle Garage Conversion ADU — Cost, Permits & Code Upgrades
Seattle has the deepest ADU policy in Washington — two units per lot, no owner-occupancy, no parking minimums, and a pre-approved DADU plan library that compresses permit time. A garage conversion ADU reuses an existing detached or attached garage shell as a legal dwelling unit — frequently the fastest and lowest-cost path to a permitted ADU in the Puget Sound. Seattle permits garage conversion adu on most single-family lots under its current code. Alley-loaded DADUs in Ballard, Beacon Hill, and Capitol Hill remain the strongest fit. AADUs above garages, basement conversions in Wallingford / Roosevelt, and pre-approved DADU plans on flatter lots compete well.
SPEC_C.01 / ZONING — Seattle, King County
Is a garage conversion allowed in Seattle?
Seattle allows garage conversion ADU on single-family lots (up to 1,000 sf), no owner-occupancy requirement, and no off-street parking required near transit.
- Max ADU size
- 1,000 sf (verify against city code)
- Parking
- No off-street parking required citywide for ADUs.
- Owner-occupancy
- Not required
- Height
- DADUs typically 18–24 ft depending on lot width and roof form; refer to SMC 23.44.041 for the current matrix.
- Setbacks
- 5 ft side/rear typical for DADUs; reduced when alley-loaded. AADUs follow primary-structure setbacks.
- Lot coverage
- Max lot coverage in single-family zones generally 35%, with the DADU footprint counted separately under city accounting.
- Minimum lot size
- No minimum lot size for an AADU; DADU eligibility commonly requires ≥3,200 sf parcel — verify per current code.
- Tree retention
- Tree Protection Ordinance (SMC 25.11) — exceptional trees and trees ≥ 24" DBH trigger arborist review.
- Critical areas
- ECA review for liquefaction-prone areas, steep slopes ≥ 40%, peat settlement zones, and riparian buffers.
- Alley access
- Alley-served lots get reduced setbacks and easier utility routing; common in Ballard, Beacon Hill, and Capitol Hill.
Feasibility blockers to map early
- liquefaction (Duwamish, Interbay)
- steep slope (West Seattle, Magnolia)
- peat (parts of Roosevelt, Northgate)
- ECA review for liquefaction-prone areas, steep slopes ≥ 40%, peat settlement zones, and riparian buffers.
- Tree Protection Ordinance (SMC 25.11) — exceptional trees and trees ≥ 24" DBH trigger arborist review.
SPEC_C.02 / PERMIT — Seattle Department of Construction & Inspections (SDCI)
Seattle garage conversion permit timeline
Plan review (typical)
10–26 weeks
Standard DADU/AADU review; pre-approved DADU plans review faster (≈ 6–10 weeks).
With one correction cycle
12–34 weeks
Includes one typical correction cycle.
Permit difficulty
7/10
Higher = more plan-check + design-review friction.
Documents you'll submit
- Site plan with parcel dimensions, structures, setbacks, and tree retention
- Architectural plans (floor plans, elevations, sections)
- Structural plans stamped by a WA-licensed engineer where required
- WSEC energy compliance worksheet
- Stormwater / drainage review per city threshold
- Utility plan (sewer / water / electric)
- Tree inventory & protection plan (SMC 25.11)
- Side-sewer card from SPU when sewer connection is altered
- Existing-conditions plan with as-built slab elevations and framing
- Foundation upgrade detail to bring perimeter up to current code
- Envelope retrofit detail for insulation and weather barrier compliance
Common plan-check corrections
- Setback or lot-coverage encroachment on site plan
- Energy-code U-value / glazing percentage mismatch
- Missing structural calcs for shear walls or beams
- Stormwater BMP not selected or sized for the impervious area
- Tree retention plan missing or critical-root-zone fencing not shown
- ECA boundary not shown when parcel is within steep-slope or liquefaction overlay
- Pre-approved DADU plan modifications exceed allowable deviation envelope
- Existing slab not verified for habitable use
- Envelope U-value below current WSEC requirement
Inspection sequence
- 01Foundation / footing
- 02Underfloor framing & insulation
- 03Rough-in: framing, mechanical, plumbing, electrical
- 04Insulation & vapor barrier
- 05Drywall / wallboard
- 06Final building, plumbing, mechanical, electrical
- 07Certificate of occupancy / final approval
Why Seattle reviews can slow
- Resubmittals for energy-code revisions on glazing or envelope
- Side-sewer capacity letter delays from SPU
- Tree-protection plan revisions when an exceptional tree is identified late
- Coordinated review for ECA parcels adding 4–8 weeks
SPEC_C.03 / COST — Seattle cost tier 5/5
Seattle garage conversion cost range
$130k–$290k · typical $200k
Existing slab + framing reuse drives the low end; new foundation + full envelope rebuild drives the high end.
What moves your number
- Existing slab condition
- Perimeter foundation upgrade
- Envelope retrofit depth
- Plumbing routing
Utility & site notes for this city
- 200A panel upgrades common; SPU side-sewer reviews can require capacity upsizing on older lines.
- Panel upgrades to 200A typically $4–8k including SCL coordination; sub-panels for DADUs $2.5–5k.
- SPU stormwater triggers above 750 sf new impervious; flow-control BMPs may be required.
- Hillside / crane access / shoring can add $30–80k; ECA review adds soft costs and weeks of timeline.
Ranges reflect typical 2026 project budgets and exclude land. Your exact cost depends on site access, foundation, utility upgrades, finish level, and city review depth — confirm with a feasibility study before committing.
SPEC_C.04 / TIMELINE — typical end-to-end ~60 weeks
Seattle garage conversion schedule, phase by phase
- 01FeasibilityParcel + zoning + utility check.1–4 weeks
- 02DesignSchematic → permit set.4–12 weeks
- 03EngineeringStructural runs in parallel with design.2–6 weeks
- 04Permit reviewStandard SDCI ADU intake.10–26 weeks
- 05Corrections & resubmittalMultiple correction cycles possible on ECA parcels.2–8 weeks
- 06ConstructionDetached scope; conversions on the faster end.14–34 weeks
- 07Final sign-offFinal inspections and CofO.1–4 weeks
Seattle-specific delay risks · ECA / steep slope review · exceptional tree designation · SPU side-sewer coordination
Weather note · Excavation, foundations, and exterior weatherization sequence best Apr–Oct; framing and finishes continue year-round under temporary weather protection.
SPEC_C.05 / COMPARE — Garage Conversion vs the other ADU types in Seattle
Why pick a garage conversion over the alternatives in Seattle?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| DADU | Lower — reused shell. | Faster — no new framing. | Comparable — often detached. | Slightly lower — smaller, lower ceilings typical. | Existing garage is sound and large enough. |
| AADU | Similar. | Similar. | Higher when garage is detached. | Comparable. | Garage is detached or has alley access. |
| Basement ADU | Often lower — fewer waterproofing risks. | Faster — drier, more accessible. | Higher when detached. | Comparable. | You'd rather not occupy the main-house basement. |
SPEC_C.06 / FIT — Is a garage conversion right for your Seattle lot?
Best-fit profile for a Seattle garage conversion
Pick a garage conversion when an existing garage is structurally sound, has workable ceiling height, and the city accepts the existing slab. Avoid it when the slab is cracked, the foundation is shallow, or the envelope cannot reach current WSEC numbers without taking the shell apart.
- Rental income
- Good for long-term rental in the right neighborhood — usually under 600 sf of livable area, which fits 1-bedroom market demand.
- Aging parents / multigen
- Good when the garage is single-level, has alley access, and can be brought to ADA-adjacent standards.
- Tight lot
- Best of the four when no new footprint is acceptable — the structure already exists.
- Alley access
- Excellent — most detached garages already face an alley with utilities nearby.
- Lower budget
- Lowest baseline budget of the four when the existing shell is sound.
- Privacy
- Strong if detached; weaker if the garage is attached to the main house.
- Speed to occupancy
- Fastest of the four when no foundation upgrade is required.
- Utilities already nearby
- Often the strongest fit — existing electrical, frequently existing water rough-in.
SPEC_C.07 / SOURCES — verified .gov / city / state references
Official sources for Seattle ADU rules
- Seattle SDCIPermit authority for all Seattle ADU work.
- Seattle ADU programCurrent ADU/DADU rules, application checklists, pre-approved DADU plans.
- Seattle Services PortalOnline intake for ADU permit applications and inspections.
- SMC 23.44.041 — Accessory dwelling unitsCode citation for accessory dwelling units in single-family zones.
- Seattle Environmentally Critical AreasLiquefaction, steep slope, peat — checked against parcels before feasibility.
- RCW 36.70A.681 — Accessory Dwelling UnitsState law setting the floor for ADU regulations cities must meet.
- HB 1337 (2023) — Bill textAuthoritative text of the 2023 ADU reform: two ADUs per lot, 1,000 sf min, no owner-occupancy, parking limits near transit.
- WA Department of Commerce — ADU resourcesState guidance and model code for cities implementing HB 1337.
Seattle Garage Conversion FAQ
Do I need to live on the lot to build an ADU in Seattle?
No. Seattle removed the owner-occupancy requirement in 2019; one or both units can be rented.
Can I build two ADUs on a single Seattle lot?
Yes. Single-family lots can host one AADU and one DADU subject to lot size, FAR, and ECA review.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
What is the maximum DADU size in Seattle?
1,000 sf of gross floor area. Garages and storage do not count, but check current SMC for accounting changes.
How long does an SDCI ADU permit take?
Standard reviews are typically 10–26 weeks. Pre-approved DADU plans land in roughly 6–10 weeks when the lot is clear of ECA constraints.
Go deeper: Read the Seattle SDCI ADU permit process, step by step (2026) guide
Will my existing garage slab pass code?
Often yes for an at-grade conversion, but the slab must be verified for vapor, depth, and (for additions of mass) bearing. A 4-inch slab over compacted base with a vapor retarder is the common minimum; older slabs sometimes need an overlay or a topping pour.
Go deeper: Read the Garage conversion timeline in Puget Sound (2026) guide
Do I have to keep the garage door?
No. Most cities require infilling the garage door opening with a framed wall and code-compliant insulation and finish. A few preserve the visual rhythm of the streetscape by requiring carriage-style cladding — confirm in design review where applicable.
Go deeper: Read the Garage conversion timeline in Puget Sound (2026) guide
Can I add a second story above the converted garage?
Sometimes — but a second story usually means a new foundation, new shear walls, and new framing. At that scope, a new DADU is often the better answer than overloading an old garage shell.
Go deeper: Read the Garage conversion timeline in Puget Sound (2026) guide
What if my garage has no plumbing?
Plumbing can be added, but the routing cost and the cost of breaking the slab to lay drain lines is one of the bigger variables in a garage conversion. Bring a plumber to the feasibility walk.
Go deeper: Read the Garage conversion timeline in Puget Sound (2026) guide
Other ADU types in Seattle
SPEC_C.08 / NEXT
Plan your Seattle garage conversion the right way
A 45-minute feasibility call confirms zoning, utility, and budget reality before you spend on design. Golden State ADU has been licensed in Washington since 2012 (LIC SEATTI*752N6).
More for Seattle garage conversion planning
INTERNAL_LINKS / Deep Map