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.

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.