Question 1

Why does cost per square foot vary by ADU size?

Accepted Answer

Fixed costs (foundation per linear foot of perimeter, kitchen, bath, MEP terminations, permit fees, mobilization) spread across more square footage as you build bigger. A 400 sq ft studio runs $380–$440/sq ft in Puget Sound 2026; a 1,000 sq ft two-bedroom drops to the $330–$380/sq ft floor. Per-sq-ft is lower on bigger units because finish, kitchen and bath aren't doubled.

Go deeper: ADU Cost Per Square Foot

Question 2

What's included in your $/sqft numbers?

Accepted Answer

All-in turn-key cost: site prep, foundation, framing, weather barrier, roofing, windows, MEP rough and finish, insulation, drywall, paint, flooring, cabinets, countertops, standard appliances, bath fixtures, side-sewer connection, electrical service upgrade if needed, permits, plan check, design fees, our overhead and profit. Not included: landscaping, fencing, solar PV, art.

Question 3

How accurate are per-sqft numbers across cities?

Accepted Answer

Within ±10% of actual closed-contract values for typical lots. Outliers — steep slope, critical areas, off-grid utilities, premium finishes — can swing 15–25% above the published band. The cost-per-sqft tables are starting points for budgeting; final number always comes from a feasibility study or pre-construction estimate for your specific site.

Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide

Question 4

Why is Seattle more expensive than Tacoma?

Accepted Answer

Three drivers: (1) modestly higher labor cost (Seattle metro construction wages 8–12% above Tacoma); (2) higher AHJ fees (Seattle plan check ~60% above Tacoma); (3) higher land/site cost (tighter lots mean more site work per sq ft). Seattle still pencils at $330–$380/sq ft for a clean 1,000 sq ft DADU; Tacoma lands $330–$360/sq ft. Build quality is identical — we run the same crews.

Go deeper: Read the Cold-weather concrete pours in the Puget Sound (2026 field guide) guide

Question 5

Do bigger ADUs cost less per square foot?

Accepted Answer

Yes, all the way to the 1,200 sq ft DADU cap. Going from 600 → 800 sq ft typically drops $/sq ft by 8–12%. Going from 800 → 1,000 drops another 5–8% and floors at $330/sq ft. The sweet spot for total ROI in 2026 is 900–1,100 sq ft.

Go deeper: ADU Cost Per Square Foot

Question 6

Where can I see the underlying data?

Accepted Answer

Every cost-per-sqft table on the site cites: (1) median of our closed contracts in the last 12 months, (2) range from 25th–75th percentile, (3) sample size, (4) last refresh date. We do not publish individual project costs without client consent. For a sanity check, ask any active ADU lender for their current $/sq ft underwriting band — they will line up close to ours.

Go deeper: Glossary: ECA (Environmentally Critical Area)

City	400 sqft	600 sqft	800 sqft	1,000 sqft	1,200 sqft
Seattle	$460–$540	$400–$460	$360–$410	$330–$380	$320–$360
Bellevue	$497–$583	$432–$497	$389–$443	$356–$410	$346–$389
Kirkland	$483–$567	$420–$483	$378–$431	$347–$399	$336–$378
Redmond	$478–$562	$416–$478	$374–$426	$343–$395	$333–$374
Tacoma	$391–$459	$340–$391	$306–$349	$281–$323	$272–$306
Sammamish	$483–$567	$420–$483	$378–$431	$347–$399	$336–$378
Issaquah	$469–$551	$408–$469	$367–$418	$337–$388	$326–$367
Renton	$428–$502	$372–$428	$335–$381	$307–$353	$298–$335
Mercer Island	$524–$616	$456–$524	$410–$467	$376–$433	$365–$410
Shoreline	$437–$513	$380–$437	$342–$390	$314–$361	$304–$342
Everett	$414–$486	$360–$414	$324–$369	$297–$342	$288–$324
Mukilteo	$446–$524	$388–$446	$349–$398	$320–$369	$310–$349
Lynnwood	$419–$491	$364–$419	$328–$373	$300–$346	$291–$328
Edmonds	$451–$529	$392–$451	$353–$402	$323–$372	$314–$353
Mountlake Terrace	$423–$497	$368–$423	$331–$377	$304–$350	$294–$331
Mill Creek	$442–$518	$384–$442	$346–$394	$317–$365	$307–$346
Bothell	$446–$524	$388–$446	$349–$398	$320–$369	$310–$349
Woodinville	$460–$540	$400–$460	$360–$410	$330–$380	$320–$360
Burien	$414–$486	$360–$414	$324–$369	$297–$342	$288–$324
SeaTac	$405–$475	$352–$405	$317–$361	$290–$334	$282–$317
Tukwila	$400–$470	$348–$400	$313–$357	$287–$331	$278–$313
Des Moines	$419–$491	$364–$419	$328–$373	$300–$346	$291–$328
Federal Way	$405–$475	$352–$405	$317–$361	$290–$334	$282–$317
Kent	$405–$475	$352–$405	$317–$361	$290–$334	$282–$317
Auburn	$396–$464	$344–$396	$310–$353	$284–$327	$275–$310
University Place	$423–$497	$368–$423	$331–$377	$304–$350	$294–$331
Lakewood	$386–$454	$336–$386	$302–$344	$277–$319	$269–$302
DuPont	$423–$497	$368–$423	$331–$377	$304–$350	$294–$331
Puyallup	$391–$459	$340–$391	$306–$349	$281–$323	$272–$306
Lacey	$382–$448	$332–$382	$299–$340	$274–$315	$266–$299
Tumwater	$382–$448	$332–$382	$299–$340	$274–$315	$266–$299
Olympia	$386–$454	$336–$386	$302–$344	$277–$319	$269–$302

ADU cost per sqft by city + size

Cost per square foot — frequently asked