A backyard cottage built around your lot, not a catalog.
When a pre-approved plan won't fit — a tight lot, a slope, a specific layout for an aging parent or a 2-bedroom rental — a custom detached ADU gives you the right building. It costs more and takes longer than a pre-approved plan, and we'll tell you exactly where the money and the months go.
Is this you?
You’ve looked at pre-approved plans and none of them fit — your lot is irregular or sloped, you need a specific accessible layout, or you want a 2-bedroom that actually pencils as a rental. You also remember the Reddit horror stories: “quoted $550K for a DADU above a garage,” “$600–700/sf from one architect, $1,000/sf from another.” You want the custom building without the custom runaround.
This format fits if you want to:
- Maximize rental income with a 1- or 2-bedroom unit sized to your lot.
- Build a single-level, accessible cottage for a parent or for your own downsize-in-place.
- Use difficult lot geometry that a standard pre-approved plan can’t accommodate.
What’s included — and what’s not
| Included | Not included |
|---|---|
| Custom design + structural engineering for your lot | The land — you already own it |
| Foundation, full vertical build, kitchen, bath, HVAC | Major utility upgrades beyond a standard connection |
| Energy-code-compliant envelope + finishes | Unusual foundation work on extreme slopes or fill (priced separately) |
| Permit package, corrections cycle, inspections | Furniture, appliances above standard, extra landscaping |
| Standard utility connection | Condoization legal / survey work (offered separately) |
Published price range
All-in = vertical build + soft costs + sitework + permits + a standard utility connection. Not the land. Standard finish, 2025–26 King County data. A premium finish moves the per-square-foot number up; steep slopes and septic add their own line items.
| Size | Vertical build | All-in (standard) |
|---|---|---|
| 600 sf 1BR | ~$175K–$240K | $240K–$335K |
| 800 sf 1–2BR | ~$230K–$325K | $320K–$450K |
| 1,000 sf 2BR | ~$290K–$405K | $400K–$560K |
This is a planning range from real project data, not a bid. Your number depends on site conditions, finish level, and design. We confirm an exact, fixed-scope price on a real quote.
How it works
- Feasibility — zoning, lot coverage, setbacks, ECA / slope, septic, utility capacity.
- Design + engineering — drawings tailored to your lot and your use.
- Permit package + submittal to SDCI or your city / King County DPER.
- Permit review + corrections — typically several months for a custom plan.
- Build (4–6 months), inspections, utility connection, move-in or rent.
City notes
Custom is the path when there’s no pre-approved program in your city, or your lot can’t use one. In Seattle, custom DADU permit review runs 4–8 months through SDCI. Kirkland allows up to 1,200 sf — the largest cap in King County — which custom design can fully use. Unincorporated King County permits through DPER, and rural zones gate detached ADUs on minimum acreage. Across King County, HB 1337 removed owner-occupancy and parking requirements, and lot subdivision (condoization) is now legal if you want the option to sell the cottage separately later.
Open-market quotes run from a $350–400/sf floor to $750–800/sf, with the occasional $1,000/sf outlier — for similar buildings. The spread is mostly opacity: which bid includes utilities, soft costs, geotech, and the permit corrections cycle, versus which one leaves them out to look cheap. Compare all-in dollars, not $/sf.
