What an ADU actually costs in Seattle & King County
You asked three contractors the same question and got three wildly different numbers — $250K, $400K, $550K — for the same backyard cottage. None of them is necessarily lying. Here are the real ranges and the line items that don't show up in the first quote.
This guide gives you the real ranges, the line items that don't show up in the first quote, and the reason identical projects come back at 2x to 4x apart. The numbers below are King County and Seattle market data, pulled from city fee schedules, the King County rate book, local builders, and real project reports — not a national calculator that assumes Phoenix labor rates.
The honest headline: a new detached DADU in Seattle runs roughly $250,000 (small, basic) to $700,000+ (large, complex site). Garage conversions are the cheap entry point at $80,000–$350,000. Attached and basement ADUs land in between. If a published number is under $350/sf, be suspicious — real Seattle projects, fully loaded, almost never come in that low.
The fast answer: total cost by project type
| Project type | Typical size | Low end | Mid range | High end | All-in cost/sf |
|---|---|---|---|---|---|
| Garage conversion | 400–600 sf | $80,000 | $155,000 | $250,000 | $150–$280/sf |
| Basement conversion (AADU) | 500–1,000 sf | $120,000 | $210,000 | $350,000 | $120–$300/sf |
| Attached AADU / addition | 400–800 sf | $200,000 | $310,000 | $475,000 | $250–$500/sf |
| New DADU — small | ~400 sf | $180,000 | $240,000 | $310,000 | $300–$500/sf |
| New DADU — medium | ~700 sf | $280,000 | $380,000 | $500,000 | $300–$550/sf |
| New DADU — large | ~1,000 sf | $360,000 | $490,000 | $700,000+ | $350–$650/sf |
| Home addition (build-out) | per sf | — | — | — | $200–$300/sf |
| Home addition (second story) | per sf | — | — | — | $300–$600/sf |
These ranges triangulate across multiple Seattle builders and real owner reports. A few real numbers from the field, for calibration:
- An r/Seattle owner got a 1,000 sf DADU built for $400,000 and called the builder "excellent."
- An r/realestateinvesting builder reported $480,000 in construction on a DADU + 2-car garage — $600,000 after taxes and builder fees.
- Multiple West Seattle contractors quoted around $300,000 for a DADU in April 2026, with one saying he's "never seen less than $500–$700/sf in practice."
"Vertical" vs. "all-in": the two numbers that aren't the same number
When a builder quotes you a price, the first question to ask is: is that vertical construction, or all-in?
- Vertical (hard) costs are the building itself — foundation, framing, roof, siding, windows, mechanical/electrical/plumbing, interior finishes. This is the number contractors like to lead with because it's the lowest defensible figure.
- All-in (project) costs add everything it takes to actually get that building permitted, connected, and finished: design and engineering, permits and impact fees, site work, utility connections, the contractor's overhead and profit, sales tax, and a contingency.
The gap between the two is enormous. A common pre-construction figure — before a shovel touches dirt — is $30,000–$60,000 in soft costs. One investor breakdown for a 750 sf DADU put full soft costs (including financing) at $50,000–$80,000. If a quote shows you a $300K vertical number and stops there, your real check will be meaningfully larger.
Roughly, on a fully loaded Seattle DADU:
- Hard construction is the bulk of it.
- Soft costs (design, permits, financing) add 15–25% on top.
- Sales tax adds ~10.25% on materials and labor.
- A 10–15% contingency is genuinely recommended, not padding.
The line items that don't show up in the first quote
This is where budgets blow up. None of the items below are exotic — they're routine — but they're routinely left off the opening number. Sorted by how often and how hard they hit.
| Hidden cost | Typical range | Why it's left out |
|---|---|---|
| Side sewer / wastewater connection | $8,000–$30,000+ | Depends on distance to the main, depth, and whether it crosses an arterial. The #1 budget surprise on Puget Sound. A sewage ejector pump adds ~$5,000 if the lot is flat. |
| SPU water service tap (3/4", 2026) | $4,800–$6,400 non-arterial; $6,175–$8,200 arterial | New SPU development charge; site-specific. |
| Electrical service / panel upgrade | $5,000–$30,000 | Many Seattle homes have 100-amp or maxed-out 200-amp panels. A detached unit often needs its own service. |
| King County wastewater capacity charge | $46.01/month in 2026 (~$8,280 over 15 years) | A perpetual monthly utility bill, not an upfront fee — so it never makes it into the project budget. |
| Geotech report (sloped lots) | $3,000–$15,000+ | Required by SDCI on slopes over 15% or in landslide-hazard areas (a lot of Seattle). If it finds bad soil, foundation cost jumps another $20K–$50K. |
| SPU System Development Charges (new Jan 1, 2026) | $5,000–$25,000+ (verify — site-specific, still phasing in) | Brand-new in 2026 for wastewater and drainage; absent from most published guides. |
| Sales tax on construction | ~10.25% of materials + labor (≈$35,875 on a $350K build) | Frequently quoted "before tax." Washington has no income tax but taxes construction contracts. |
| Permit revisions / re-review | $292/hr (SDCI 2026 rate) | Plan review is hourly above the minimum. Two correction cycles can add $1,500–$5,000+. |
| Tree protection / arborist | $1,500–$15,000+ | Seattle's 2023 tree ordinance protects "exceptional" trees ≥24". Report, fencing, or removal permit all add up. |
| Landscaping / site restoration | $2,000–$25,000 | Construction destroys the yard; restoration is rarely in the base bid. |
| Stormwater / GSI | $5,000–$20,000 if triggered | Engineered systems (rain garden, infiltration trench) kick in past impervious-surface thresholds. |
| Water main extension (worst case) | $50,000–$375,000 (rare) | If the main doesn't front your lot, SPU can require you to extend it. Extreme outlier — but check it in feasibility. |
The one that catches the most people: the King County capacity charge is billed forever, like a utility, and it rarely appears in any ROI math. Over a 15-year hold it's roughly $8,280 you didn't plan for.
How permit fees actually break down
Permits aren't the biggest cost, but they're where the 2025–2026 increases hit, and where Seattle and unincorporated King County diverge sharply.
| Fee item | Amount | Source |
|---|---|---|
| Seattle SDCI: 500 sf DADU plan review + permit (2025) | $2,908 | SDCI 2025 Fee Summary |
| Seattle SDCI: same DADU (2026) | $3,453 (+18.7%) | SDCI 2026 Fee Summary |
| King County DPER: new ADU >500 sf (2025) | $1,424 base | KC 2025 Fee Guide |
| King County DPER: addition/conversion to living space (2025) | $2,721 base (+49% vs. 2024) | KC 2025 Fee Guide |
| School impact fee — DADU (Seattle SD #1) | ~$7,400/unit | per SDCI |
| School impact fee — attached ADU | ~$3,700/unit | per SDCI |
| Total permit + impact fees, Seattle (all-in) | $10,000–$20,000 | local builders |
| SDCI base hourly rate (2026) | $292/hr | SDCI 2026 |
Why one project gets quoted 2x to 4x apart
Homeowners post bids to Reddit constantly asking "is this reasonable?" — because the spread makes no sense to them. Here's what's actually driving it.
1. Network rate vs. open-market rate. Builders who specialize in ADUs and run pre-approved plans quote around $350–$400/sf. Search "ADU builder" cold on Google and you'll see $650–$950/sf, with open-market averages around $750–$800/sf and outliers at $1,000/sf. Same building. Different pipeline.
2. Pre-approved vs. custom. A pre-approved plan can cut design cost ~40% and shrink permitting from months to weeks. A custom design on a constrained lot costs more on both ends.
3. The quote is incomplete. The lowest bid is very often the one missing line items — utilities, sewer, geotech — that resurface mid-build as change orders. A $80,000 gap between bids is usually a $80,000 gap in scope, not efficiency. The advice that shows up over and over in the forums: get 3–5 bids, and never take the lowest one on price alone.
4. The site. A flat lot with sewer at the curb and a 200-amp panel is a different project than a steep lot needing geotech, a new electrical service, a 100-foot sewer run, and a protected tree in the way. Utility runs beyond the baseline add roughly $200 per linear foot — a 100-foot run is $20,000 by itself.
5. Finish level. Rental-grade interior finishes run $25K–$45K on a medium DADU. Premium finishes run $80K–$130K+. That's a $90,000 swing inside the same footprint.
City variation: where you build changes the math
| Factor | What it does to your budget |
|---|---|
| Seattle | ADUniverse pre-approved plans (fast lane); new 2026 SPU charges; tree ordinance; Seattle City Light power-connection wait up to 6 months. |
| Kent | Allows shared utilities — can save ~$40,000 vs. separate connections. |
| Auburn | 3-year property-tax exemption on the added value; but garage conversions must re-finish the exterior to match the house. |
| Sammamish | School impact fee $10,914/unit — budget for liquidity. |
| Kirkland | Allows 1,200 sf DADUs (largest in the county) and has a pre-approved plan catalog. |
| Renton | Free PRADU pre-approved plans (8 designs); converts non-conforming garages. |
| Unincorporated KC | Permit fees up 49% since 2025; septic and Public Health review; no pre-approved program. |
Conversions: what's inside the number
Garage conversion to living space
| Line item | Cost range |
|---|---|
| Demo / insulation removal | $2,000–$5,000 |
| Slab prep (topping or insulated subfloor) | $5,000–$15,000 |
| Framing (partitions, ceiling) | $8,000–$20,000 |
| Roof / weatherization | $5,000–$15,000 |
| Windows + exterior doors (egress required) | $4,000–$12,000 |
| MEP (electrical upgrade often needed) | $20,000–$45,000 |
| Interior finishes | $15,000–$35,000 |
| Typical total | $80,000–$250,000 |
What's usually not in the quote: foundation repair if the slab is failing, separate electrical service, plumbing tie-in fees, and the ADU permit/impact fees themselves.
Attached AADU / basement conversion
| Line item | Cost range |
|---|---|
| Structural / egress (egress window $2K–$5K each) | $10,000–$40,000 |
| Waterproofing / moisture control (critical in the PNW) | $8,000–$25,000 |
| MEP (full kitchen + bath) | $20,000–$50,000 |
| Interior finishes | $20,000–$50,000 |
| Permits + design | $8,000–$20,000 |
| Typical total | $120,000–$350,000 |
A realistic timeline (because time is a cost)
Cost isn't only dollars — it's the carry on a construction loan while you wait. Plan for:
- Pre-approved DADU permit: 2–6 weeks (Seattle, best case, simple lot).
- Custom DADU permit: 4–8 months (SDCI); 8–14 weeks (King County DPER).
- Seattle City Light power hookup: up to 6 months — a real, recent delay that can stall an otherwise-finished project.
- Construction: 4–9 months.
- Total, realistic: 9–18 months from plan to move-in. The most common regret on the forums is "it took twice as long as I was told."
Every number here is a range because every lot is different. We give you a fixed, line-item bid after we've checked your actual site — sewer, slope, panel, trees, and all. Open the cost calculator → · Which ADU type is right for you? →
A new detached DADU typically runs $300,000–$700,000+ all-in depending on size and site. Garage conversions start around $80,000; basement and attached ADUs run $120,000–$475,000. The all-in number includes design, permits, utilities, tax, and contingency — not just the building.
