CAP_15 / HOW-TO / SEATAC
How to build an ADU in SeaTac, WA
The full sequence in 12 honest steps — feasibility, design, 8–12 wk of permitting, foundation, framing, finish, and Certificate of Occupancy. Roughly 9 months end-to-end at $374k all-in for a typical SeaTac 800 sqft DADU.
The 12 steps, in order
STEP_01
1wk
Feasibility & site analysis
We pull the SeaTac parcel report, GIS overlays, and UL-7,200 / UL-12,000 code to confirm setbacks, lot coverage, height limits, and any ECA/critical-area triggers. Outcome: a 1-page go/no-go with maximum buildable envelope on your SeaTac lot.
STEP_02
3wk
Schematic design & client review
Two to three floor-plan options sized to your goals (rental income, multi-gen, home office). We pair each with an exterior massing and a target budget anchored to SeaTac's current trade-bid environment.
STEP_03
4wk
Construction documents & engineering
Structural, MEP, energy-code (WSEC 2021), and any geotech reports required for SeaTac (faa part 77 surfaces and aircraft noise (nem 65–75 db) cover most of the city). Stamped CDs are what gets submitted, not preliminary plans.
STEP_04
10wk
SeaTac permit submittal & review
Intake with SeaTac permitting, response to correction cycles, and any required side-sewer / right-of-way / tree-protection sub-permits. SeaTac currently averages 8–12 wk on ADU review.
STEP_05
2wk
Mobilization & site work
Erosion control, tree-protection fencing, demolition (if any), excavation, utility trenching. 7,200 sqft lot minimum applies for new DADUs.
STEP_06
2wk
Foundation
Forms, rebar, inspection, pour, cure. Stem-wall or slab-on-grade per the soils report. Standard spread footings work on most lots.
STEP_07
4wk
Framing, roof & weather-in
Floor, walls, roof, sheathing, windows, and dried-in envelope. From here the structure is protected from Puget Sound weather and trades can sequence in parallel.
STEP_08
3wk
Rough MEP & inspections
Plumbing, electrical, HVAC (typically heat-pump mini-split for SeaTac), low-voltage. Each scope passes its rough-in inspection before insulation.
STEP_09
4wk
Insulation, drywall & exterior
WSEC-compliant insulation, vapor control, drywall hang/finish, siding (Hardie/wood/metal), and roofing. Exterior finish locks down the envelope's long-term durability.
STEP_10
4wk
Interior finish, fixtures & cabinets
Flooring, paint, doors, trim, cabinets, counters, plumbing fixtures, light fixtures, appliances. Where most of the "luxury feel" budget actually shows up.
STEP_11
1wk
Final inspections & Certificate of Occupancy
SeaTac final building, electrical, mechanical, plumbing inspections; energy-code verification; address assignment; CofO issued. Once issued, the unit can legally be occupied or rented.
STEP_12
2wk
Punch list, warranty & lease-up
30/60/90-day touch-ups. We hand over a warranty binder (manuals, model numbers, Golden State ADU Builders Inc 2-year workmanship, 10-year structural). If renting, we provide SeaTac comparable rent data for the listing.
Building an ADU in SeaTac — straight answers
How long does it take to build an ADU in SeaTac?
Roughly 9 months end-to-end on a typical SeaTac lot — about 10 weeks of permitting (SeaTac currently averages 8–12 wk) plus 30 weeks of design, site work, framing, and finish. Lots with ECA, geotech, or shoreline overlays add 4–8 weeks.
What does an 800 sqft DADU cost in SeaTac?
Around $374k all-in for a mid-spec 800 sqft detached ADU in SeaTac — design, permits, construction, utility connections. Garage conversions run 35–45% lower, attached ADUs (AADU) about 10–15% lower.
Do I need an architect, a designer, or just a contractor in SeaTac?
Most SeaTac ADUs under 1,000 sqft can be permitted with a stamped designer set; full architectural service is optional but valuable on tight or sloped lots. We carry both in-house, so the client doesn't pay for a coordination layer they don't need.
Can I live in the main house while the ADU is being built in SeaTac?
Yes for detached ADUs (DADUs) — the main house stays habitable the entire build. Attached ADUs and garage conversions involve 2–6 weeks of louder/dustier work near the main house, but the main house remains occupiable throughout.
What's the riskiest part of building an ADU in SeaTac?
FAA Part 77 surfaces and aircraft noise (NEM 65–75 dB) cover most of the city — that's where SeaTac projects most often slip schedule or pop the contingency. We sequence geotech, arborist, and side-sewer scope before permit submittal so surprises surface during design (cheap) rather than after foundation (expensive).
What permits do I need to build an ADU in SeaTac?
Master Use Permit (where applicable), building permit, mechanical, plumbing, electrical, side-sewer connection, and any tree/right-of-way/shoreline sub-permits. SeaTac permitting packages most of these under a single ADU intake.