SPEC_C / SEATAC · AADU · Verified 2026-01-15
SeaTac AADU Builders — Attached ADU Permits, Cost & Layout
SeaTac is a small but transit-rich ADU market built around the airport and the Tukwila International Boulevard / Angle Lake light-rail stations. An AADU is a self-contained dwelling carved into the existing house — its own entry, its own kitchen, its own bath, with a fire-rated demising wall between units. SeaTac permits attached adu (aadu) 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 aadu allowed in SeaTac?
SeaTac allows attached ADU (AADU) 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 aadu 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)
- Demising-wall and fire-separation detail between primary and accessory units
- Egress plan showing independent entry for the AADU
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
- Fire-separation detail incomplete between units
- Egress missing for the accessory unit
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 aadu cost range
$158k–$334k · typical $229k
Existing-space conversions sit at the low end; new additions at the high end.
What moves your number
- Demising-wall scope
- Existing structural capacity
- Egress / window scope
- Mechanical separation
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 ~58 weeks
SeaTac aadu 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.18–38 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 — AADU vs the other ADU types in SeaTac
Why pick a aadu over the alternatives in SeaTac?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| DADU | Lower — reused shell. | Faster — less new structure. | Lower — shared wall. | Slightly less separation; still a permitted second dwelling. | Lot is tight or the main house already has separate-entry potential. |
| Garage Conversion | Similar — depends on existing-conditions scope. | Similar — both reuse a shell. | Better — usually inside the main house, separate entry. | Higher — counts as living area in the main house. | Existing house has unused basement or side wing. |
| Basement ADU | Slightly higher — AADU often touches more of the main envelope. | Similar. | Comparable. | Comparable. | You want a side or upper-level unit, not just a basement. |
SPEC_C.06 / FIT — Is a aadu right for your SeaTac lot?
Best-fit profile for a SeaTac aadu
Pick an AADU when the existing house has the structural capacity, separate-entry potential, and ceiling height to absorb a second unit. Avoid it when reusing the existing envelope would compromise fire separation, egress, or sound transmission below code.
- Rental income
- Strong for long-term rental, especially when the primary unit will be owner-occupied — shared envelope keeps operating costs low.
- Aging parents / multigen
- Excellent — proximity inside the same building, no weather exposure between units, easier caregiving access.
- Tight lot
- Best of the four on a tight lot — no new footprint, no new setbacks to fight.
- Alley access
- Less relevant — AADU entries usually face the street or side yard, not the alley.
- Lower budget
- Lower baseline than DADU — reused envelope, reused roof, reused foundation.
- Privacy
- Lower than DADU — shared wall, sometimes shared mechanical chase or service entry.
- Speed to occupancy
- Faster than DADU — less weather exposure, less framing, no roofing scope.
- Utilities already nearby
- Strong fit — typically extends existing service rather than running new lines.
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 AADU 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)
Do I need a separate electrical meter for an AADU?
Not in most Puget Sound cities. A single meter is acceptable as long as the panel has capacity, the AADU has dedicated branch circuits, and any required sub-panel is sized correctly. Separate meters are usually only required if you plan to bill the AADU tenant for utilities directly.
How thick does the demising wall have to be?
Most plan reviewers require a 1-hour fire-rated separation between dwelling units, with STC 50 sound transmission. The exact assembly depends on the existing wall — UL-listed assemblies and city-approved alternatives are both accepted, but the listing must be on the drawings.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
Can the AADU share an entry with the main house?
Most cities require a separate, independent entry for the AADU at grade. A shared vestibule with two interior doors is sometimes acceptable, but a shared interior hallway usually is not.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
Does an AADU need its own HVAC?
Each dwelling unit must have an independent temperature-control source. A dedicated mini-split or fan-coil for the AADU is the common solution — central forced-air shared across both units is generally not allowed under the current Washington Energy Code.
Other ADU types in SeaTac
Same aadu in nearby cities
SPEC_C.08 / NEXT
Plan your SeaTac aadu 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 aadu planning
INTERNAL_LINKS / Deep Map