How Much Does It Cost to Build an ADU in Seattle in 2025?

How much does it cost to build an ADU in Seattle? For a new detached ADU (a DADU or backyard cottage), plan on a total in the mid-six figures once design, SDCI permits, utility connections, site work and construction are all counted, while a basement or garage conversion usually costs less because part of the structure already exists. The real number for your lot depends on a handful of line items, and the ones that surprise homeowners most are the site costs that national cost guides never mention.

We have been building in Seattle since 1989, with more than 100 DADUs and custom homes completed across King and Snohomish County. Below is how we break down an ADU budget, so you can see where the money goes before you ever ask for a bid.

What drives the total

Four things set the range before anything else:

• Type of ADU. A new detached cottage costs the most. An attached addition comes next. Converting a basement or garage usually costs the least, unless the existing structure needs major upgrades.
• Size. Bigger costs more, but not proportionally. Kitchens, bathrooms, utilities and permits cost about the same whether the unit is small or large, so cost per square foot often goes up on very small units.
• Your site. Slope, soil, access, trees and how far the unit sits from your existing sewer and power lines can swing a budget more than finishes do.
• Finish level. Cabinets, counters, flooring, fixtures and appliances are where you have the most control.

For current DIGS pricing by size and type, see our realistic costs page. For a quick estimate based on your own choices, try the ADU cost calculator.

Line by line: where the money goes

Design, survey and engineering

Every permitted ADU in Seattle needs drawn plans, and most need a site survey to confirm setbacks and lot coverage. Structural engineering is standard, and you need energy code documentation under the Washington State Energy Code. If your lot sits in an Environmentally Critical Area, such as a steep slope or landslide-prone zone, SDCI will typically ask for a geotechnical report, which adds both cost and time.

Seattle also offers pre-approved DADU plans through SDCI. They can shorten review, but they still have to be adapted to your lot, utilities and foundation.

SDCI permit fees

Seattle Department of Construction and Inspections fees are calculated largely from the project's valuation, plus plan review and inspection charges. On top of the building permit, expect separate permits for the side sewer, electrical, plumbing and mechanical work. We don't quote a flat permit number because it scales with the size and value of your unit, but it belongs in your budget from day one rather than as an afterthought.

Side sewer, water and power

This is the line item that most often separates a realistic budget from a hopeful one.

• Side sewer. A DADU can often tie into your existing side sewer, but only if that line is in good shape and the grade works. We camera-scope the line early. Old clay pipe, root intrusion or a backyard that sits lower than the street can mean replacing the line or adding a pump system.
• Sewer capacity charge. King County charges a capacity charge for new sewer connections, and new ADUs are typically subject to it. It can be paid up front or over time, but it is real money that rarely shows up in online estimates.
• Water. You can usually branch off your existing service, or request a separate meter from Seattle Public Utilities if you want separate billing.
• Power. Depending on your house's current electrical service, Seattle City Light may need to upgrade it, or you may run a separate service. Either way, trenching distance matters: a cottage at the back of a deep lot costs more to connect than one near the house.

Stormwater and drainage

A new roof and new hard surfaces trigger Seattle's stormwater code. Depending on your soil, that may mean infiltration, dispersion or a connection to an approved drainage point. Seattle's glacial till often drains poorly, so this line can range from simple to significant.

Site prep and foundation

Excavation, haul-off of spoils, and the foundation itself are where lot conditions show up in the budget.

• Access. An alley or wide driveway lets equipment in. A narrow side yard can mean smaller machines, hand work and extra days.
• Slope. Sloped lots may need retaining walls, stepped footings or deeper excavation.
• Trees. Seattle's tree protection rules can require an arborist report and can limit where you build or how you dig near a protected tree.

Shell and systems

Framing, roofing, windows, siding, insulation and the mechanical systems make up the core of the build. Current energy code pushes new units toward efficient electric heat pumps and specific ventilation requirements. Attached units also need fire separation from the main house.

Finish level

This is where you decide what kind of home the ADU will be. A rental unit built for durability can use simpler cabinets and hard-wearing flooring. A unit for a parent or as your own future downsize might justify better fixtures and accessibility features. We help clients spend where it lasts and save where it doesn't.

Sales tax

Washington applies retail sales tax to construction, and in Seattle the combined rate is over 10 percent. National cost guides almost never include it, and it is one of the biggest gaps between online estimates and real Seattle bids.

Hidden costs national guides leave out

If you are comparing a number you found online to a Seattle bid, check whether it includes:

1. Washington sales tax on construction
2. Side sewer inspection, repair or replacement
3. King County sewer capacity charge
4. Stormwater management for new roof and hard surfaces
5. Electrical service upgrades and long utility trenches
6. Limited site access and spoils haul-off
7. Arborist reports and tree protection
8. Geotechnical reports on slopes or critical areas
9. Paths, fencing and landscaping restoration after the build
10. A higher property assessment once the unit is finished

Conversion or new build?

A garage or basement conversion uses an existing structure, which usually lowers cost. But it is not automatically cheap. Existing slabs often lack insulation, ceiling heights may fall short, and older foundations sometimes need work. Utility connections and permits still apply. The only way to know whether a conversion actually saves money on your property is to look at it in person.

How to get a real number for your lot

1. Start with the calculator. Use the ADU cost calculator to see how size and finish level move the total.
2. Check real pricing. Our realistic costs page shows what DIGS projects cost, not national averages.
3. Get a site visit. Sewer, slope, access and trees can only be priced by walking the lot.
4. Compare bids carefully. Make sure every bid covers the same scope. Our guide on how to compare ADU bids walks through what to look for.

Talk to a Seattle ADU builder

DIGS Construction handles design, permits, site work and construction under one roof, and owner Eric Dexter walks every site himself. If you want a real budget for an ADU on your Seattle lot, including the site costs most estimates miss, book a free consultation and we will walk through your property with you.