What a backyard ADU really costs — the honest version.
Real ranges for King County, what moves the number, and why two identical floor plans can differ by six figures. No bait numbers.
Free lot check first · real ranges, no bait numbers
The realistic range for a turnkey detached ADU.
We build turnkey detached ADUs for about $290 per square foot in King County — so a 2-bedroom cottage (Seattle and many cities now allow a DADU up to 1,200 sq ft) lands roughly in the $230K–$350K range. The exact number for your project comes after the free lot check.
| Type | Size | Typical turnkey range (~$290/sq ft) |
|---|---|---|
| Compact studio / 1-bed | ~600 sq ft | $165K–$195K |
| 2-bedroom detached ADU | 800–1,200 sq ft | $230K–$350K |
| Garage / basement conversion (AADU) | varies | from ~$90K |
Six things that move the price — and by how much.
The same floor plan can swing six figures depending on your lot. Here's what actually drives it.
Size & layout
Bigger units cost less per square foot than compact ones, but more in total. Each extra bathroom adds a meaningful chunk — plumbing, fixtures, waterproofing.
Finish level
After size, finish level is the single biggest cost lever. Electric baseboard vs a ducted mini-split, builder windows vs black-frame, laminate vs quartz, standard vs designer kitchen — the same box can cost wildly different amounts depending on how it's finished.
Utility connections
The most underestimated lines. Sewer or septic, a water meter, a power panel — trenching runs $10K–$30K+; a septic lot may need a system revision for an added bedroom (five figures, sometimes the gate factor); a 200-amp panel upgrade, a second meter, or a PSE transformer are common. Distance from the street to your backyard matters more than people think.
Site conditions
A sloped lot can add a five-figure premium (retaining, deeper foundations, harder access). Tree-retention rules, stormwater, demolition and tight machine access all move the number. A remote backyard unit can even trigger fire-access or sprinkler requirements, depending on your city.
Garage & extras
A garage, a deck, landscaping, a fully separate driveway — all real line items that are easy to leave out of a "cheap" quote and add up fast.
Your city's code & design path
Exterior-material standards, design review and extra studies vary by city. In Seattle, choosing a pre-approved DADU plan instead of a fully custom design cuts both design cost and review time.
Why the "$160K quote" usually isn't.
Three questions to ask about any low bid:
- Does it include design and permitting, or just construction?
- Does it cover every finish through move-in — and sales tax?
- Is it a fixed price, or an estimate that grows 20–30%?
The costs outside construction (so nothing surprises you).
Design & permitting — our flat-fee package (see the estimate →). City fees — set by your city, scaled to valuation. Surveys or geotech when a lot requires them. Washington sales tax — roughly 10%+ in King County, real money on a project this size; we itemize it, never bury it. And contingency — plan 10–15%, more for conversions or sloped lots.
The most underestimated of all: utility connections — sewer or septic, a water meter, a power panel. Distance from the street to your backyard drives the trenching cost more than people expect.
What comes back.
Rent: backyard units on the Eastside typically ask $1,800–$3,500/mo in long-term rent — typical asking rents, not a projection. Resale: an ADU can add six figures to a property's value, and appraisers and lenders now count them. Sell separately: Washington law now allows ADUs to be sold as condominiums in many cases; the process and cost depend on your city.
Market data, not a projection or financial advice — rents and values vary by location and unit.
The pricing questions everyone asks.
Still have questions?
Talk to an ADU builderNo pressure — a real number starts with a free lot check.
Get my free feasibility study → (425) 394-8849Why won't you just give me an exact price?
What's the cost per square foot?
Is a conversion cheaper than a new build?
The only exact number is the one after a free lot check.
Send your address and we'll run the free feasibility study — then quote a number we're prepared to build to. Real ranges now, no bait.
- Free desktop feasibility study
- A real number for your lot, not a generic estimate
- We reply within one business day
- Prefer to talk? (425) 394-8849
Got it — your feasibility study is in the queue.
We'll reply within one business day. Need to talk sooner? Call (425) 394-8849.