The price is on the page.
Almost no King County ADU builder publishes a price — most make you book a call to hear a number. We don't. Here are the all-in ranges we plan against — and the line items a low bid quietly leaves out — so you can budget before you talk to anyone.
Every number below is all-in: the vertical build plus soft costs, sitework, permits, and a standard utility connection. It is not the land — you already own that. These are planning ranges from 2025–26 King County project data, not a bid. Your real number depends on site conditions, finish level, and design. We confirm an exact, fixed-scope price after we’ve checked your actual lot.
All-in ranges by format
Standard finish, typical size for each format. A premium finish moves the per-square-foot number up; the custom DADU row shows both because it’s the format where finish swings the budget most.
| Format | Typical size | Vertical build | All-in |
|---|---|---|---|
| Garage conversion | ~450 sf | ~$85K–$145K | $115K–$200K |
| Pre-approved DADU | 400–800 sf | — | $250K–$400K |
| Custom DADU (standard finish) | ~800 sf | ~$230K–$325K | $320K–$450K |
| Custom DADU (premium finish) | ~800 sf | ~$325K–$415K | $450K–$575K |
| AADU / basement conversion | ~650 sf | ~$140K–$215K | $195K–$300K |
| Home addition | ~500 sf | ~$115K–$175K | $160K–$240K |
The pre-approved DADU range comes from Seattle’s pre-approved plan catalog rather than a per-square-foot model: you trade design flexibility for a permit in 2–6 weeks instead of months. Only a minority of the market uses it.
Vertical vs all-in: the two numbers that aren’t the same
When a builder quotes you, ask one question first: is that vertical, or all-in? Vertical (hard) cost is the building itself — foundation, framing, roof, windows, mechanical, finishes. It’s the lowest defensible figure, so it’s the one people lead with. All-in adds everything it takes to permit, connect, and finish that building. On a typical project the vertical build is roughly 72% of the all-in cost — the rest is real, and it arrives whether or not it was in the opening number.
What a low bid leaves out
When two bids on the same project come back $80K apart, that gap is almost always scope, not efficiency. These are the line items that don’t show up in the cheapest opening number and resurface mid-build as change orders.
- Land.You already own it — but a low bid sometimes quietly assumes work the site needs and the budget doesn’t cover.
- SPU water service tap.A new development charge for the water connection — site-specific, and higher on an arterial. A water-main “No Tap” situation is the worst case.
- Major utility upgrades beyond a standard connection. Side-sewer runs, electrical service or panel upgrades for a detached unit, stormwater systems if you cross an impervious-surface threshold.
- Permit fees and the King County jump. Permits run through Seattle SDCI or, in unincorporated areas, King County DPER — where fees rose 49% on January 1, 2025.
- General-contractor overhead and profit.A real GC margin is in the all-in number; a vertical-only quote hasn’t added it yet.
- Washington sales tax. Roughly 10.25% on materials andlabor — and often quoted “before tax.”
- Unusual foundation work on extreme slopes or fill, and geotech where SDCI requires it on steep or critical-area lots.
- Furniture, appliances above the standard set, and landscaping beyond restoring what construction disturbs.
Every figure here is a range because every lot is different — sewer distance, slope, panel capacity, trees. We give you a fixed, line-item bid after we’ve checked your actual site. Get a number for your lot → · See how the project runs →
Because the spread between bids on an identical project routinely runs 2x to 4x, and homeowners can't tell why without a starting point. Published ranges let you sanity-check any quote you get — ours or anyone's — before you've spent a dollar. It's the single thing virtually no King County builder will do.
