Seattle, WA · COST_DATA_2026
600 sqft ADU cost in Seattle
Real construction numbers from our 2023–2025 Seattle bid archive, scaled to 2026. A 600 sqft 1 bedroom · 1 bath · galley kitchen ADU built turn-key.
Cost per sqft
$400–$460
Total construction
$240K–$276K
Seattle permit fee
$10K
All-in (top of range)
$305K
Line-item breakdown
How a 600 sqft ADU in Seattle actually prices out. Numbers reflect $460/sqft at the top of the range; lower spec drops to $400/sqft.
| Line item | % of total | Seattle cost |
|---|---|---|
| Design + structural engineering | 8% | $22K |
| Seattle permits + plan review | 4% | $10K |
| Site prep, foundation, utilities | 16% | $44K |
| Framing + weather envelope | 22% | $61K |
| MEP rough-in (electrical/plumbing/HVAC) | 14% | $39K |
| Insulation + drywall | 6% | $17K |
| Finishes (floors, cabinets, counters, tile) | 14% | $39K |
| Appliances + fixtures | 5% | $14K |
| Landscape + final inspections | 4% | $11K |
| GC overhead + margin | 11% | $30K |
Compare 600 sqft ADU cost across Puget Sound
Same plan, same finishes — labor, permits, and trade rates drive the spread.
Mercer Island, WA
$456–$524/sqft
$274K–$314K all-in · +14% vs Seattle
Bellevue, WA
$432–$497/sqft
$259K–$298K all-in · +8% vs Seattle
Kirkland, WA
$420–$483/sqft
$252K–$290K all-in · +5% vs Seattle
Burien, WA
$360–$414/sqft
$216K–$248K all-in · -10% vs Seattle
Tukwila, WA
$348–$400/sqft
$209K–$240K all-in · -13% vs Seattle
Renton, WA
$372–$428/sqft
$223K–$257K all-in · -7% vs Seattle
Get a fixed price for your Seattle build
$1,500 feasibility: lot study, Seattle zoning verification, and a fixed-price contract you can finance against.
Request Seattle feasibilityFrequently asked
What does an ADU actually cost in Seattle, WA?
2026 Seattle ADU all-in cost (design, permits, build, side-sewer, electrical service, finishes) ranges $300–$400/sqft. A 400 sqft studio averages $120K–$160K. A 600 sqft 1BR averages $180K–$240K. A 900 sqft 2BR averages $270K–$360K. Turn-key numbers from our 2024–2025 Seattle bid archive scaled to 2026 indices. The spread is driven by soils, lot access, panel/service upgrade requirements, and finish tier — not hidden contractor markup.
Why does my Seattle ADU cost more than a friend's elsewhere?
Three cost drivers vary materially: (1) permit + SDC fees — Seattle's system development charges can swing $4–14K depending on water/sewer capacity; (2) energy code path — IECC R406 prescriptive vs. performance, where Seattle reviewers prefer a path adding $3–6K; (3) stormwater — lots near critical drainage basins trigger detention vaults at $8–22K. Construction labor is within 6% across King/Pierce/Snohomish. The "cheaper city" usually has lower SDCs, not lower build cost.
What is included in your Seattle ADU price?
Site prep, foundation, framing, weather barrier, roofing, windows, doors, MEP rough + finish, insulation, drywall, paint, flooring, cabinets, quartz counters, range/fridge/dishwasher/washer-dryer, bath fixtures, lighting, side-sewer connection from property line, electrical service upgrade if required, gas connection if applicable, blower-door test, final cleaning, one-year workmanship warranty. Excluded: landscape, fencing (priced separately), solar PV (optional), furniture, primary-house exterior repair.
Can I get a fixed price for my Seattle ADU?
Yes. After feasibility and 60% design, we issue a fixed-price contract with an itemized schedule of values. Price holds for owner-initiated scope; field-discovered conditions (rotted sheathing, buried oil tank, unmapped sewer rerouting) are priced at cost +10% with photo documentation. Average Seattle ADU sees 4–7 change orders totaling 3–6% of contract value — well below the industry norm of 8–12%. Fixed price only works because feasibility surfaces variables that would otherwise become "discovered" later.
How do Seattle permit and impact fees affect total cost?
Plan into your Seattle budget: building permit ($2,800–$6,500 by valuation), water meter capacity fee ($3,500–$11,000 if upsizing from 3/4" to 1"), sewer capacity charge ($2,400–$8,800), school impact fee ($0–$5,200 — Seattle historically at the low end for ADUs ≤1,000 sqft), and transportation impact fee where applicable. Total fees typically $9K–$28K. RCW 36.70A.681 now caps impact fees on ADUs at 50% of single-family in many jurisdictions, reducing Seattle ADU fees ~25% vs pre-2024.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What Seattle ADU finishes give the best return?
Highest-ROI upgrades for long-term Seattle rentals: quartz over laminate (+$2.8K, +$95/mo rent), full-tile shower over insert (+$3.5K, +$60/mo), in-unit washer-dryer (+$1.8K, fills 11 days faster), LVP plank over carpet (+$1.4K, halves turn cost), and a dedicated mini-split per zone (+$3.2K, +$45/mo + better reviews). Skip: high-end appliances, waterfall edges, statement lighting — no measurable rent or occupancy lift in the Seattle long-term market.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
How much does design cost for a Seattle ADU?
Catalog plan: $4,800 flat (12 pre-engineered options 380–980 sqft, pre-Seattle-approved details). Light custom (catalog plan + minor reconfiguration): $8,500–$12,000. Full custom design: $14,000–$24,000 depending on lot complexity, with structural PE stamp included. All design packages include energy compliance worksheet, drainage report coordination, and Seattle permit submittal. Design cost is roughly 4–7% of total project — under-pricing design is the cheapest way to over-pay construction.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What does excavation and foundation cost in Seattle?
Slab-on-grade on flat Seattle lots: $26K–$42K including footings, vapor barrier, and frost protection. Pier-and-beam (sloped lots, expansive soils, or where preserving a tree's critical root zone matters): $44K–$72K. Crawlspace foundation: $38K–$58K. Geotech-driven upgrades (over-excavation, controlled fill, helical piers): add $8K–$28K. Pre-1980 Seattle neighborhoods over glacial till usually fall in the slab range; alluvial soils near waterways usually need pier or helical.
How much does framing cost for a Seattle ADU?
Framing labor + lumber for a 600 sqft Seattle DADU: $32K–$48K. 2x6 advanced framing with R-23 cavity, OSB sheathing, Tyvek WRB, 24" o.c. studs to maximise insulation continuity. Roof framing: stick-built or pre-engineered trusses; trusses save 1.5–2.5 weeks of schedule for ~$1,800 more. Seattle lumber prices in 2026 are stable after the 2022 spike — Douglas fir #2 SPF studs run $3.85–$4.40 each delivered.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What does plumbing cost for a Seattle ADU?
Plumbing rough + finish for a 1-bath Seattle ADU: $22K–$32K including PEX home runs, ABS DWV, water heater (heat-pump preferred for IECC compliance, $3,400 installed), tub/shower, toilet, two sinks, dishwasher, washing machine hookup. Side-sewer tap and connection: separate trade, $4,800–$9,400 depending on trench length and depth. WA L&I requires specialty plumbing contractor — we sub to licensed PL with 5+ years of Seattle permit history.
What does electrical cost for a Seattle ADU?
Electrical rough + finish: $18K–$28K for a 100A sub-feeder from existing panel, dedicated ADU panel, LED recessed throughout, GFCI/AFCI per NEC, EV-ready conduit if requested. Service upgrade to 200A on the primary panel (often triggered by ADU + EV charger + heat pump): +$4,800–$9,200 including new mast, meter base, ground rods, and Seattle utility coordination. WA L&I licensed EL contractor required; we sub to electricians with active Seattle permit history.
How much does HVAC cost in a Seattle ADU?
Standard: ducted mini-split heat pump (Mitsubishi/Daikin) — $11K–$16K installed for a 1-zone system, $14K–$21K for 2-zone. Ducted central heat pump: $18K–$26K. Seattle's climate (~4,800 HDD/yr) makes a heat pump the right answer 95% of the time — meets IECC R403 system efficiency without electric resistance backup. ERV for IAQ (required for tight envelopes ≤3 ACH50): $3,200–$4,800 installed.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What does insulation and drywall cost in Seattle?
Insulation package to meet Seattle's 2021 WSEC: $8K–$13K for R-21 cavity (mineral wool or BIB fiberglass), R-49 attic blow-in, R-15 underfloor, air sealing per IECC R402.4. Drywall hang/tape/finish: $11K–$17K for 5/8" Type X throughout (gives fire rating + sound STC 45+). Level 4 finish standard; level 5 (skim coat) for raking light walls adds $1,400.
How much does cabinetry and countertops cost in Seattle?
Mid-range stock cabinets (Diamond, Mid-Continent, KraftMaid) with quartz tops, range/dishwasher cutout: $14K–$22K installed. Semi-custom (CliqStudios, Wellborn) at the same finish tier: $22K–$32K. Custom local shop: $34K–$55K. Bath vanity adds $1,800–$3,400. Quartz counters (Caesarstone, Silestone, MSI Q) at $68–$92/sqft installed. We discourage natural stone in tenant kitchens — etches and stains drive turn cost.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What does flooring cost in a Seattle ADU?
LVP plank (luxury vinyl plank) installed: $5.50–$8.50/sqft including underlayment and base — best ROI for Seattle long-term rentals. Engineered hardwood: $9–$14/sqft installed. Tile (bath, mudroom): $12–$22/sqft installed. Polished concrete (when slab is finish floor): $4–$8/sqft. Carpet has effectively zero application in Seattle ADUs — moisture, allergies, and turn cost all argue against.
How much does paint and trim cost in Seattle?
Interior paint (Sherwin-Williams ProMar 200 or equivalent, 2-coat) + trim package (4½" base, 2¼" casing, simple shaker doors): $14K–$22K for a 600 sqft ADU. Exterior paint over LP SmartSide siding: $8K–$12K. Door hardware (Schlage, Kwikset SmartKey, lever sets): $1,200–$1,800. Ceiling fan + bath fan + LED retrofit: included in electrical line.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What's the cost difference between 1- and 2-bedroom Seattle ADUs?
One bedroom (~600 sqft): $180K–$240K all-in. Two bedroom (~850 sqft): $260K–$340K all-in. The marginal $80K–$100K covers: additional 250 sqft of envelope (~$32K), second bedroom and closet ($6K), upgraded HVAC to 2-zone ($3K), enlarged septic/sewer demand allocation (~$4K), and design fee uptick (~$3K). Per-sqft, the 2BR is slightly cheaper because fixed costs spread further. Pencils better for long-term rental yield in Seattle.
Go deeper: Read the Seattle MFTE for ADUs: do you qualify? (2026) guide
What does landscaping and fencing cost for a Seattle ADU?
Stripped out of base price because every lot is different. Realistic Seattle ranges: code-required pathway from public way (concrete or permeable pavers) $2,400–$4,800; restoration grass + native shrub buffer $3,400–$6,800; 6 ft cedar fence (privacy from primary) $42–$58/lineal ft installed; irrigation drip system $1,800–$3,400. Most clients defer landscape to post-CO and self-perform 60–70% of it over the first season.
What hidden costs blow up Seattle ADU budgets?
Five recurring "hidden" cost surprises: (1) water-meter upsize from 3/4" to 1" at $3,500–$11,000; (2) side-sewer slope failure forcing trench rebuild $6–18K; (3) 200A service upgrade $4,800–$9,200; (4) stormwater detention vault when impervious tips threshold $8–22K; (5) exceptional-tree review forcing redesign $4–12K. Our feasibility study surfaces all five in two weeks, before you commit to design. None are actually "hidden" — they're predictable when you check.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
What contingency should I budget for a Seattle ADU?
10% on fixed-price contracts, 15% on cost-plus or pre-1940 renovation scopes. The 10% covers owner-driven scope creep + field-discovered conditions normalized across our Seattle bid archive. Above 15% suggests inadequate feasibility — money is better spent on better feasibility (geotech, sewer scope, panel calc) than on a fat contingency you'll spend chasing surprises.
How does Seattle ADU cost compare year-over-year?
Seattle ADU all-in $/sqft from our internal index: 2022 $268, 2023 $312 (+16%, lumber + labor squeeze post-COVID), 2024 $336 (+8%), 2025 $352 (+5%, normalizing), 2026 projected $362 (+3%). Material costs have stabilized; labor remains the upward pressure as Seattle's carpenter labor pool ages without enough apprenticeship throughput. Plan on 3–4%/yr inflation for medium-term Seattle ADU pro-forma underwriting.
Go deeper: Read the Seattle MFTE for ADUs: do you qualify? (2026) guide
INTERNAL_LINKS / Deep Map