How Much Does an ADU Cost in Seattle? (2026 Price Guide)

• Conversion ADU: $85k–$160k · 300–650 sq ft · 3–5 months (garage or basement).
• Detached Cottage (DADU): $185k–$320k · 400–900 sq ft · 5–7 months of construction.
• Premium Custom: $320k+ · 800–1,200 sq ft · 7–10 months of construction.
• Typical detached range: most Seattle-area DADUs we deliver fall between $250,000 and $450,000 all-in.
• End-to-end timeline: plan on 10–16 months from first conversation to keys — feasibility + design + permits + build.
• Fixed-price contract: one number covers design, permits (including city fees), and construction — no draws for line-item surprises.
• Built by a design-build team with 93+ ADUs completed across King, Pierce, and Snohomish counties.

We publish three package tiers on the pricing page so you can benchmark your project before we ever meet. Each tier is a fixed-price contract — design, permitting, and construction all inside one number.

Most of the backyard DADUs we build finish in the $250,000–$450,000 window once size, finishes, and site conditions are set.

One number, one contract, three phases handled by the same team:

• Design: feasibility, site plan, floor plans, elevations, and the construction drawings the city reviews.
• Permits: the full permit package plus all city fees — SDCI review, plan check, and utility connection fees roll into the contract price.
• Construction: demo, foundations, framing, MEP, insulation, drywall, finishes, and final inspections through certificate of occupancy.

What you write on the contract on day one is what you pay on day the ADU is finished. No surprise draws for scope that should have been priced in.

The package range exists because three qualitative levers move the price within a tier:

• Site conditions: slope, sewer condition and run length, panel capacity, tree protections, and access. Before you price a build, confirm the parcel pencils — will your lot qualify for a DADU walks through the five factors that shape site cost. If the property is on a septic system, the bedroom-capacity review comes first — see building an ADU on septic in King or Pierce County.
• Size and layout: square footage, one story vs two, and whether the floor plan is a repeat-friendly layout or a fully custom one.
• Finishes: cabinetry, counters, flooring, fixtures, appliances, and windows. Same footprint, very different finished cost.

Zoning and legal capacity are set by state and city rules — see the 2026 Washington ADU laws for what you're now allowed to build on a Seattle-area lot.

The construction window on the package chart is only part of the story. From the first conversation to a certificate of occupancy, plan on 10–16 months total: feasibility and design, then permitting (the least predictable phase), then the 3–10 months of construction the tier calls for. Starting the design and permit work early is the single biggest thing you can do to shorten the calendar.

Two steps, both free:

1. Run your parcel through the Analyze My Lot tool for a 60-second read on zoning, buildable area, and a ballpark cost band.
2. Book a free site visit and we'll confirm sewer, slope, and access, then quote a fixed number against one of the three packages.

Once you have a number, the next decision is who to hire against it — a design-build specialist, a general contractor you trust, or a prefab vendor — and each path has a different effect on that fixed-price line. Our ADU companies vs general contractors comparison walks the nine trade-offs that decide which vendor type fits which project.