Seattle ADU & DADU Cost Guide (2026): what a backyard cottage really costs
If you've searched "DADU cost Seattle," you've seen numbers from $150,000 to half a million — which is useless when you're trying to decide whether to build. This guide breaks the cost down the way a builder actually estimates it, names the five site conditions that move the price most, and shows you how to gauge whether the rent will justify the spend.
The honest 2026 range: $250,000–$450,000
For a detached accessory dwelling unit (DADU) — a true standalone backyard cottage — most Seattle projects land between $250,000 and $450,000 all-in. "All-in" matters: that figure includes architectural design, the full Seattle Department of Construction & Inspections (SDCI) permit set, structural and energy-code engineering, utility connections, and finished construction ready to rent or move into. A 1-bedroom cottage of roughly 500–650 square feet sits near the bottom of that band. A 2-bedroom or near-1,000-square-foot unit with a real kitchen and a comfortable bath sits near the top.
Why so high for something small? Because a DADU is a complete building, not a bump-out. It needs its own foundation, roof, weather envelope, electrical service, plumbing, a kitchen and a bathroom — the same expensive systems a 2,500-square-foot house has, just compressed onto a tiny footprint. That's why the per-square-foot number ($400–$600+) looks alarming next to a remodel even though the total is far lower.
Where the money actually goes
On a typical Seattle DADU budget, the rough breakdown looks like this:
| Cost area | Share of budget | What's in it |
|---|---|---|
| Site, foundation & structure | ~25–30% | Excavation, foundation, framing, seismic |
| Envelope & roof | ~15% | Rainscreen siding, roofing, windows, flashing |
| Mechanical, electrical, plumbing | ~20% | Heat pump, panel, fixtures, utility tie-ins |
| Kitchen, bath & finishes | ~20% | Cabinets, counters, flooring, trim |
| Design, permits & fees | ~10–15% | Architecture, SDCI, utility & impact fees |
Prices in USD. Shares vary by site and finish level; your written fixed-scope bid follows a free consult.
The five things that swing the price most
Two identical floor plans on two different lots can differ by $80,000 or more. Here's what causes that:
- 1
Side-sewer condition & capacity
This is the most common hidden cost. If your existing side sewer is undersized, root-clogged, or lacks the slope to add a second unit, you may need a new side-sewer line to the main — a five-figure surprise that has nothing to do with the cottage itself.
- 2
Site access & slope
A flat lot with alley access is cheap to build on. A tight, steep, or alley-less lot means hand-digging, pumped concrete, retaining walls, and crews hauling material through a narrow side yard — all of which add labor.
- 3
Soils & foundation type
Seattle's glacial till is great; its liquefaction-prone fill and steep-slope soils are not. A geotechnical report that calls for pin piles or a deeper foundation can add tens of thousands.
- 4
Finish level
Builder-grade vs. designer kitchen and bath is the single most controllable variable. The same shell can carry a $20,000 kitchen or a $55,000 one.
- 5
Trees & critical areas
An exceptional tree, a steep-slope overlay, or a drainage easement can shrink your buildable footprint or force the cottage into a more expensive spot on the lot.
How to check feasibility before you spend on design
Before paying an architect, confirm what your specific lot allows. This is the five-step process we run in a feasibility study:
- Pull your zoning & lot data. Confirm zone, lot area, and any environmentally critical area overlays (steep slopes, liquefaction, watercourses).
- Run the development standards. Apply rear-yard coverage, the height limit, and Seattle's 1,000-square-foot DADU floor-area cap to find your maximum footprint.
- Check trees & critical areas. Map exceptional trees and tree-protection zones, plus slope and drainage constraints.
- Verify utilities & side-sewer capacity. Confirm water, power, and side-sewer slope and capacity — the make-or-break utility item.
- Model the budget & ROI. Combine buildable size with current per-foot cost and local rent comps for a realistic all-in number and payback estimate.
Will it pay for itself?
Many Seattle owners rent a DADU for roughly $1,800–$2,800 per month depending on size and neighborhood, and the added square footage meaningfully lifts property value. Whether that pencils out depends on your build cost, your financing rate, and how long you plan to hold the property. We dig into those numbers in our companion DADU rental income guide. For the rules, sizing and build process, see our Seattle ADU & DADU service page — and when you're ready, request a free estimate and we'll model your lot.
Seattle ADU cost — common questions
How much does a DADU cost in Seattle in 2026?
A detached backyard cottage typically runs $250,000–$450,000 all-in (design, SDCI permits, utilities and construction). A 1-bed near 500–650 sq ft lands near the low end; a 2-bed or 800–1,000 sq ft unit near the high end. Hard sites, high-end finishes, and side-sewer replacement push it higher.
Why is a Seattle ADU pricey per square foot?
A DADU is a complete standalone building. The fixed costs of a foundation, roof, envelope, separate utilities, a kitchen and a bath spread over a small footprint, so the per-foot figure is high even though the total is lower than a big remodel.
What drives ADU cost the most?
Site access and slope, side-sewer condition and capacity, the foundation type your soils require, finish level, and tree or critical-area constraints. Two identical plans on different lots can differ by $80,000+.
Is a DADU a good investment here?
Often yes — many owners rent one for $1,800–$2,800/month and gain property value. Whether it pays off depends on build cost, financing and hold time, which a feasibility study should model first.