How Much Does It Cost to Build a DADU in Seattle?

A DADU — a Detached Accessory Dwelling Unit, or backyard cottage — is one of the smartest ways to add real living space, rental income and long-term value to a Seattle-area property. But the first question almost every homeowner asks is the same: what does it actually cost to build one?

Here's an honest answer from a builder who's completed 100+ of them across King and Snohomish County — the real 2026 price ranges, the per-square-foot math, and the three cost categories that decide your final number.

The short answer: what a DADU costs in Seattle (2026)

For a fully permitted, professionally built DADU in the Greater Seattle area, plan for roughly:

Those aren't teaser numbers — they're the ranges we quote real clients. Where your project lands inside that range comes down to three things: soft costs, site development, and hard construction costs. You can see the full breakdown on our realistic costs page, or estimate your own project with our ADU cost calculator.

What actually drives the cost of a DADU

Every DADU budget breaks into three buckets. Understanding them is the difference between a realistic budget and a nasty mid-project surprise.

1. Design & permitting (soft costs) — ~10–15%

Roughly $40–$80 per square foot: architecture, structural engineering, geotechnical reports, and the city permit process. In Seattle and King County, permitting is often the single longest part of the timeline — and starting from a proven design gets you to submission sooner. Browse our ready DADU floor plans.

2. Site development — ~15–30%+

Roughly $50–$150+ per square foot, and this is the wild card. A flat lot with utilities nearby is cheap to prep. A sloped lot, an Environmental Critical Area (ECA), poor soils or a long utility run can add tens of thousands. It's exactly why we run a paid pre-construction feasibility study before quoting a firm price — so the number you get is real, not a guess. See how that works in our process.

3. Construction (hard costs) — ~70–85%

Roughly $250–$480 per square foot: the actual build — foundation, framing, mechanical/electrical/plumbing, insulation and finishes. Finish level is where you have the most control; the same footprint can be built to a clean, efficient standard or fully custom.

Does a bigger DADU cost less per square foot?

Usually, yes — up to a point. Fixed costs like the kitchen, a bathroom, utility hookups and permitting get spread over more square footage, so a 1,000 sq ft DADU often has a lower per-foot cost than a 500 sq ft one. Most Washington jurisdictions allow DADUs up to 1,000–1,200 sq ft, and our best-selling plans land right around 1,000 sq ft for exactly this reason.

How to keep your DADU on budget

• Start from a proven plan where you can — custom design is beautiful but adds soft cost and review time.
• Get a real feasibility study before you fall in love with a design. Site surprises are the #1 budget-buster.
• Choose your finish level intentionally — splurge where you'll feel it daily, save where you won't.
• Work with one team that self-performs the sitework, foundation and build — fewer handoffs, fewer markups, fewer delays.

Is a DADU worth it?

For most Seattle-area homeowners, yes. A well-built DADU adds a rentable, income-producing unit, increases resale value, and gives you flexible space for family — all on land you already own. Many of the DADUs we build across the Greater Seattle area are cash-flow positive from the first tenant.

Get a real number for your lot

Every lot is different, which is why we don't quote averages. Tell us about your property and goals and we'll give you a clear, line-item estimate — no hidden fees, no pressure. Owner-led from the first call through final inspection.

Related reading: How to finance an ADU in Seattle, ADU vs DADU — what's the difference? and Do you need a permit to build a DADU in Seattle?