SPEC_C / SEATAC · GARAGE CONVERSION · Verified 2026-01-15
SeaTac Garage Conversion ADU — Cost, Permits & Code Upgrades
SeaTac is a small but transit-rich ADU market built around the airport and the Tukwila International Boulevard / Angle Lake light-rail stations. 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. SeaTac permits garage conversion adu on most single-family lots under its current code. AADUs above garages and basement conversions outperform tall DADUs on Part 77-constrained lots.
SPEC_C.01 / ZONING — SeaTac, King County
Is a garage conversion allowed in SeaTac?
SeaTac 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 within ½-mile of major transit per HB 1337; otherwise city standard parking rules apply.
- Owner-occupancy
- Not required
- Height
- DADU heights typically capped near 24 ft and further limited by FAA Part 77 airspace surfaces near the airport.
- Setbacks
- 5 ft side/rear typical for DADUs in most residential zones.
- Lot coverage
- Lot coverage typically 40–50% by zone; verify per parcel.
- Minimum lot size
- No explicit minimum lot size under HB 1337 alignment.
- Tree retention
- Tree-retention rules apply above thresholds; many lots are open with limited canopy.
- Critical areas
- Airport noise contours and FAA Part 77 surfaces are the dominant overlays; some parcels in the McMicken / Riverton drainage carry stream buffers.
- Alley access
- Alleys uncommon citywide.
Feasibility blockers to map early
- airport noise overlay (citywide)
- FAA Part 77 height surfaces
- older sewer/septic transitions
- Airport noise contours and FAA Part 77 surfaces are the dominant overlays; some parcels in the McMicken / Riverton drainage carry stream buffers.
- Tree-retention rules apply above thresholds; many lots are open with limited canopy.
SPEC_C.02 / PERMIT — SeaTac Community & Economic Development
SeaTac garage conversion permit timeline
Plan review (typical)
8–18 weeks
SeaTac Community & Economic Development standard ADU intake.
With one correction cycle
10–26 weeks
Includes one typical correction cycle.
Permit difficulty
5/10
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)
- 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
- 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 SeaTac reviews can slow
- Port of Seattle / FAA height-clearance coordination on tall DADUs
- noise-mitigation construction review
- sewer-district verification
SPEC_C.03 / COST — SeaTac cost tier 3/5
SeaTac garage conversion cost range
$114k–$255k · typical $176k
Slab + framing reuse vs full envelope rebuild drives the range.
What moves your number
- Existing slab condition
- Perimeter foundation upgrade
- Envelope retrofit depth
- Plumbing routing
Utility & site notes for this city
- Service split between Highline Water/Sewer and Midway; confirm provider per parcel before design.
- Panel upgrades typically $4–7k including PSE meter swap.
- Side-sewer reviews routine; older septic-to-sewer conversions still occur on legacy parcels in unincorporated-historic zones.
- Sound-attenuation construction in noise-contour zones can add $10–25k.
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 ~54 weeks
SeaTac garage conversion schedule, phase by phase
- 01FeasibilityParcel + zoning + utility verification.1–4 weeks
- 02DesignSchematic → permit-ready set.4–12 weeks
- 03EngineeringStructural runs in parallel with design.2–6 weeks
- 04Permit reviewSeaTac Community & Economic Development standard ADU intake.8–18 weeks
- 05Corrections & resubmittalCycle depends on correction depth.2–8 weeks
- 06ConstructionDetached scope; conversions sit at the lower end.12–32 weeks
- 07Final sign-offFinal inspections and certificate of occupancy.1–4 weeks
SeaTac-specific delay risks · Port of Seattle / FAA height-clearance coordination on tall DADUs · noise-mitigation construction review · sewer-district verification
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 SeaTac
Why pick a garage conversion over the alternatives in SeaTac?
| 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 SeaTac lot?
Best-fit profile for a SeaTac 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 SeaTac ADU rules
- SeaTac Community & Economic DevelopmentPermit authority for SeaTac ADUs.
- SeaTac ADU informationLanding page; ADU rules under SMC Title 15.
- MyBuildingPermit.comShared permit portal for SeaTac.
- SeaTac Municipal CodeSMC chapters with ADU standards and airport overlays.
- 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.
- WA State Energy Code (Residential)ADUs must meet WSEC; the residential provisions drive insulation, glazing, and HVAC requirements.
SeaTac Garage Conversion FAQ
Does SeaTac allow two ADUs per lot?
Yes — under HB 1337 alignment SeaTac permits one AADU and one DADU on most single-family lots.
Is owner-occupancy required in SeaTac?
No. SeaTac removed the owner-occupancy requirement to align with state law.
How does the airport affect my DADU height?
FAA Part 77 imaginary surfaces extend from the runways and can cap structure height well below the city's zoning maximum. The Port of Seattle / FAA review confirms allowable height per parcel.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
Do I need noise-attenuation construction?
If your parcel falls within the airport noise contour, yes — additional wall/roof assemblies, sealed glazing, and mechanical ventilation are required for habitable space.
Go deeper: Glossary: RRIO (Rental Registration & Inspection Ordinance)
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 SeaTac
Same garage conversion in nearby cities
SPEC_C.08 / NEXT
Plan your SeaTac 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 SeaTac garage conversion planning
INTERNAL_LINKS / Deep Map