ADU Builders in Seattle
Attached ADUs and backyard DADUs, planned, permitted and built by one licensed team.
- We check what your lot allows before anyone draws a plan.
- We prepare the drawings and handle every permit the unit needs.
- You approve an itemized, written price before work starts.
By appointment. Limited start dates.
- Over 200 projects
- 4.9 on Google
- 20+ years building
- WA license VOLCABI792C6
Why Seattle homeowners add a second home on the lot
An ADU is a complete, separate home with its own kitchen, bathroom and locking door, on the same lot as your house. People call us about one for three reasons.
Family needs a place close by
A parent who should not live alone, an adult child saving for a first home, a caregiver. An ADU keeps them next door, with privacy on both sides.
The yard or basement is doing nothing
A lower floor used for storage, a detached garage full of boxes, a deep backyard. That space can become a rental or room for the next stage of family life.
The rules feel like a maze
Zoning, size limits, a separate entrance, fire separation, sewer and power. We sort out what your lot can take first, so you decide with the facts in hand.
Attached ADUs, DADUs and conversions
Seattle names two kinds of accessory dwelling unit. An attached ADU (AADU) sits inside or against your house. A detached ADU (DADU), often called a backyard cottage, is its own building on the lot. We plan and build both, and we convert existing space into either.
Attached ADUs
A basement, lower floor or new wing turned into a separate unit. Seattle requires its own locking entrance, apart from the main door, and fire and sound separation from the main house unless interconnected smoke alarms run through the whole building.
Detached ADUs (DADUs)
A new backyard cottage on its own foundation, with living, sleeping, kitchen and bathroom space. Its heating controls must be separate from the main house, and it gets its own smoke and carbon monoxide alarms.
Garage and structure conversions
An existing detached garage or accessory building can become a DADU. A converted structure must fully meet the Seattle Energy Code, so the insulation, windows and heat usually change completely.
Kitchen and bathroom
Every ADU needs a full kitchen and a bathroom, the two rooms our crews build most often. The Renton kitchen on this project page took 4 weeks on site.
Utilities and service
A new unit usually means altered electrical circuits under a separate electrical permit, new plumbing under a Public Health plumbing permit, and a side sewer permit when it ties into your existing side sewer. We coordinate each one with Seattle City Light.
Design, plans and permits
We draw the site plan, floor plans, energy calculations and structural plans, show you the design in 3D and file the permit.
What homeowners say about working with us
Just completed my inital meeting with Volcano Builders about my bathroom remodels and couldn't be happier with the interaction. On time, professional and extremely knowledge. Highly recommend you invite them out to see your next home improvement project!
We were nervous about hiring a contractor because we were first-time homeowners, but Volcano Builders made it easy to understand. They made everything clear and helped us stick to our budget. I'm very happy with how everything turned out.
Overall, we had a great time with Volcano Builders. There were a few small delays, but they were clear in their communication and kept things moving. The last kitchen remodel turned out great, and the quality seems like it will last for a long time.
Bar and the team at Volcano Builders did a great job! Professional service, great communication throughout the whole process, and everything was done with care and attention to detail. Highly recommend!
I had a great time with Volcano Builders.The team was professional and on time, and the kitchen turned out just how we wanted it.
Volcano did a master bath remodel for us. The project was completed efficiently and on time. All of their workman were courteous and hard working. When we had a couple of material delivery delays they kindly adjusted their schedules. Their pricing was fine and the terms of the contract were fulfilled. We highly recommend Volcano Building.
How an ADU project works
Three steps, one team, and a written price before anyone picks up a tool.
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Lot check and feasibility
We measure the lot and the house, check zoning, sewer and power, and tell you which type of ADU the site can take.
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Design, price and permit
We design the unit with you, show it in 3D, price it in writing line by line and file the drawings, then answer any correction letter the city sends.
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Build and final inspection
One project manager runs the foundation, framing, utilities and finishes through each inspection. After the final inspection is approved, the unit can be lived in.
What drives the cost of an ADU in Seattle
An ADU is a small house, and four things set most of its price. Your written estimate prices each on its own line.
Attached, detached or converted
Converting a basement or garage reuses walls, a roof and often a foundation. A new DADU needs all three built from scratch, plus the path and utility runs across the yard.
Size and layout
Seattle caps an ADU at 1,000 square feet. Within that, a compact plan with the kitchen and bathroom sharing a wet wall uses less pipe, less framing and less finish than a spread out one.
Sewer, water and power
How far the unit sits from the main line, whether the panel and service can carry a second home, and whether a new side sewer is needed all change the site work. King County also bills a sewer capacity charge for every new ADU.
Site, code and finishes
A slope, tree roots, tight access for equipment and the fire separation an attached unit needs all add work, and so do the finishes you choose.
Two choices save the most. Check feasibility before paying for custom drawings, because the lot decides more than the design does. And look at the pre-approved DADU plans on the city's ADUniverse site: we can build from one.
How Much Does It Cost to Build an ADU?
Financing is available: Home Renovation Financing in Seattle.
Homeowners on camera
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Stephanie
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Linda, Part 1
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Michelle
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Mrs. Sergeant
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Paul & Burley
ADUs and DADUs across Greater Seattle
Our main office is in Bellevue, and we build ADUs, DADUs and conversions across King and Snohomish counties, from Seattle to Kirkland, Redmond, Issaquah and Lynnwood.
Each city writes its own ADU rules. In Seattle, the Seattle Department of Construction and Inspections issues the building permit and checks zoning, and DADUs are not allowed on lots in shoreline districts. In Bellevue, permits come from Development Services. We confirm the rules for your address at the first visit.
Planning a remodel of the main house as well? See our remodeling pages for Seattle and Bellevue, or all 132+ cities we serve.
ADU and DADU questions
What is the difference between an ADU and a DADU?
An ADU is any accessory dwelling unit: a separate home with its own kitchen, bathroom and entrance on the same lot as a house. In Seattle, an attached ADU (AADU) is inside or part of the house, and a detached ADU (DADU) is a separate building such as a backyard cottage. Neither can be sold off separately from the main house. Our guide ADU vs DADU in Seattle, WA: What Is the Difference? compares them side by side.
How many ADUs can I build on a Seattle lot, and how big?
Seattle allows two ADUs on a lot in addition to the main house, in any mix of attached and detached, and each can be up to 1,000 square feet. Setbacks and height for an ADU are no stricter than for the house itself. Zoning, critical areas and shoreline rules can still limit a lot, so we check yours first.
Do I need a permit for an ADU in Seattle?
Yes. SDCI treats a unit built without a permit as not legal. Building a new unit or adding one inside your house takes a construction permit, and legalizing an existing unit takes a permit to establish use. Expect a separate electrical permit, a plumbing permit from Public Health, Seattle and King County, and a side sewer permit if the unit ties into your side sewer. See Do You Need a Permit to Remodel in Seattle and Bellevue?
Does an ADU need its own parking space?
No. Seattle does not require parking for an ADU. What you cannot do is remove a required off street parking space to make room for the unit unless you replace it elsewhere on the lot.
Can I turn my garage or basement into an ADU?
Often, yes. A basement becomes an attached ADU when it gets its own locking entrance, a kitchen, a bathroom, an egress window in each bedroom and fire and sound separation from the house above. A detached garage can become a DADU if it can meet the Seattle Energy Code in full. Read Garage Conversion and Designing an ADU for layout ideas.
Is an ADU or a home addition right for us?
An ADU is a second, independent home, suited to a parent, an adult child or a renter. A home addition makes your own house bigger and shares its kitchen and entry. See Home Additions if the extra space is for your household.
Can I see ADUs you have built?
Our ADU projects are not in the online portfolio yet, so we show you comparable work at the consultation. Online, you can see the kitchens and bathrooms our crews build, the two rooms every ADU needs, in the kitchen remodel before and after and bathroom remodel before and after portfolios, including the Bellevue bathroom remodel and a converted attic in the Kirkland attic remodel.
How do I get a price?
Book a consultation. We visit, check the lot and the house, and talk through who the unit is for, then send a written, itemized price before any work starts. For background, read How Much Does It Cost to Build an ADU? and the Seattle and Bellevue Remodeling Cost Guide.
Book your ADU consultation
Four quick steps. Tell us about your project and we’ll reach out to discuss it and see if we’re the right fit.