SPEC_C / SEATAC · DADU · Verified 2026-01-15
SeaTac DADU Builders — Detached ADU Cost, Permits & Timeline
SeaTac is a small but transit-rich ADU market built around the airport and the Tukwila International Boulevard / Angle Lake light-rail stations. A DADU is a standalone backyard cottage on the same lot as the main house — separate roof, separate entry, full kitchen and bath. SeaTac permits detached adu (dadu) 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 dadu allowed in SeaTac?
SeaTac allows detached ADU (DADU) 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 dadu 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)
- DADU site plan showing separation from primary structure and any easements
- Independent utility connections (sewer, water, electric) or shared-service detail
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
- Setback or coverage error on detached structure
- Stormwater BMP missing for added impervious
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 dadu cost range
$246k–$458k · typical $334k
Drivers: site access, foundation, utility upgrades, and finish level.
What moves your number
- Foundation type (slab vs stepped)
- Crane / site access
- Sewer routing length
- Finish level
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 dadu 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 — DADU vs the other ADU types in SeaTac
Why pick a dadu over the alternatives in SeaTac?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| AADU | Higher — full new foundation + envelope. | Slower — ground-up build. | Better — no shared walls. | Strongest resale separation. | Backyard space + alley/side access available. |
| Garage Conversion | Higher — no reused shell. | Slower — full new structure. | Better — siting flexibility. | Higher — true second dwelling. | Lot can absorb a new footprint. |
| Basement ADU | Higher — no reused shell. | Slower — full new structure. | Much better — separate building. | Higher — visible second unit. | You want a true backyard cottage, not a downstairs unit. |
SPEC_C.06 / FIT — Is a dadu right for your SeaTac lot?
Best-fit profile for a SeaTac dadu
Pick a DADU when privacy, long-term rentability, and resale value matter more than speed or lowest cost. Avoid it on very tight lots, lots with deep critical-area buffers, or sites with no alley access and a long sewer run.
- Rental income
- Strongest of the four for long-term rental income — full privacy, full appliance set, dedicated yard space and parking when allowed.
- Aging parents / multigen
- Excellent for aging parents who want independence with proximity — single-level layouts with no shared walls or stairs.
- Tight lot
- Hardest of the four on a tight lot — DADU footprint plus required setbacks and tree retention can consume most rear-yard space.
- Alley access
- Ideal when alley access exists — separate guest entry, easier construction staging, and cleaner utility tie-ins.
- Lower budget
- Highest baseline budget of the four — foundation, framing, roof, siding, and full MEP are all new.
- Privacy
- Best privacy of all four — no shared walls, no shared entry, no shared mechanical chase.
- Speed to occupancy
- Slowest of the four — full ground-up build, full inspection sequence, full weather exposure during framing.
- Utilities already nearby
- Sewer routing length and side-sewer condition drive a meaningful share of the budget.
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 DADU 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.
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
Which utility district serves my SeaTac lot?
Service is split between Highline Water/Sewer and Midway. Confirm the provider before design.
Are pre-approved DADU plans accepted in SeaTac?
SeaTac does not run its own pre-approved DADU plan library. Standard custom or stock-plan submittals run through the normal review path.
Go deeper: Read: Seattle's Pre-Approved DADU plans — when they make sense
Do I need a separate sewer line for a DADU?
Not always. Most Puget Sound cities allow a shared side-sewer with a cleanout and an as-built that proves capacity. A separate line is required only when the existing line is undersized, in failing condition, or the city explicitly demands one.
Can I rent the DADU short-term?
Short-term rental rules are city-specific and change. Seattle, Bellevue, Tacoma and most Tier-1 cities allow long-term rentals; short-term (STR) usually requires a separate license, an owner-occupancy attestation, or both. Confirm with the city before underwriting an STR pro forma.
How big can a DADU be in Washington?
HB 1337 sets a statewide floor of at least 1,000 sq ft of allowed gross floor area for an ADU. Individual cities may allow more (Seattle and Bellevue both go above 1,000 sf on many lots), but the city's specific max controls — always verify against the linked municipal code.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
What kind of foundation does a DADU need?
A stem-wall foundation on continuous footings is most common in the Puget Sound, with stepped foundations on sloped sites. Pin or pier foundations are sometimes used on tree-protection lots to limit critical-root-zone disturbance, but they require engineering.
Go deeper: Read the Foundation inspection checklist for Puget Sound homes guide
Other ADU types in SeaTac
Same dadu in nearby cities
SPEC_C.08 / NEXT
Plan your SeaTac dadu 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 dadu planning
INTERNAL_LINKS / Deep Map