Cost of an ADU or DADU in the Greater Seattle Area
ADUs we build across the Greater Seattle Area start at $150,000+ for a garage or basement conversion and $250,000+ for an attached ADU. Detached DADUs start at $325,000+ for a small cottage and $425,000+ for a 600 to 1,000 sq ft unit.
Prices set by Bar Unger, owner of Volcano Builders, Inc. WA contractor license VOLCABI792C6. Updated October 2026.
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Transparent pricing
ADU and DADU Pricing by Type and Finish Level
Price depends on three things: whether the unit is converted, attached or detached, its size and number of baths, and the finish level. Pick a type to see a typical footprint and what each finish level includes.
Garage and Basement Conversion ADUs
A legal unit inside an existing structure
Typical size: 300 to 800 sq ft · studio or 1 bedroom, 1 bath
Common layouts: Garage to ADU · Basement to ADU · Studio over a garage
Typical timeline: 3 to 5 months on site, after design and permits
Basic
Functional + affordable
Starting at
$150,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Insulation, drywall and code egress windows
- Kitchen with standard cabinets and quartz
- One bath with a tub or shower
- Separate locking entrance
- Fire and sound separation from the main house
- LVP flooring and ductless heat pump
Best for: A legal unit in space you already have
Mid-Range
Style + durability
Starting at
$200,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Everything in Basic
- Semi-custom kitchen cabinets
- Tiled walk-in shower
- New exterior door and entry
- Laundry hookups
- Lighting plan
Luxury
Design + premium finish
Starting at
$275,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Custom cabinetry and stone counters
- Large format tile and frameless glass
- Heated bath floor
- Upgraded windows and acoustic separation
- Level entry and wider doors on request
- Full selections with our designer
Super-Luxury
No limits
Starting at
$375,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Full custom millwork and built-ins
- Slab stone surfaces
- Spa level bath
- Smart lighting and controls
- Premium windows and doors
- Dedicated design and project management
Design and permits: drawings, structural engineering, survey and permit fees are priced separately in the design phase. See design and permit services. Appliance purchase and installation are not included. We prepare the connections.
Not included in starting prices: Design, structural engineering, survey and permit fees · City plan review and utility connection fees · King County sewer capacity charge · New electrical service, water meter or side sewer · Long utility trenches, sewer pump, rock or retaining walls · Tree removal, arborist or geotech reports · Fire sprinklers where the city requires them · Appliance purchase and installation · Material upgrades beyond the allowance. We price these in writing if your project needs them.
Attached ADUs
A new wing with its own entrance
Typical size: 400 to 1,000 sq ft · 1 or 2 bedrooms, 1 bath
Common layouts: Ground floor wing · Unit over a new garage · Side addition
Typical timeline: 5 to 8 months on site, after design and permits
Basic
Functional + affordable
Starting at
$250,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Foundation, framing and roof tied into the house
- Siding and roofing matched to the house
- Kitchen and 1 bath
- Separate locking entrance
- Fire and sound separation from the main house
- LVP flooring and ductless heat pump
Best for: A separate unit joined to the house
Mid-Range
Style + durability
Starting at
$325,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Everything in Basic
- Covered entry porch
- Kitchen with quartz counters
- Tiled walk-in shower
- In-unit laundry
- Lighting plan
Luxury
Design + premium finish
Starting at
$425,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Vaulted or raised ceilings where the roof allows
- Custom cabinetry and stone counters
- Premium windows and exterior details
- Heated bath floor
- Layered lighting and smart controls
- Full selections with our designer
Super-Luxury
No limits
Starting at
$550,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Two bedrooms and two baths
- Full custom millwork
- Slab stone surfaces
- Private patio or deck
- Premium windows and doors
- Dedicated design and project management
Design and permits: drawings, structural engineering, survey and permit fees are priced separately in the design phase. See design and permit services. Appliance purchase and installation are not included. We prepare the connections.
Not included in starting prices: Design, structural engineering, survey and permit fees · City plan review and utility connection fees · King County sewer capacity charge · New electrical service, water meter or side sewer · Long utility trenches, sewer pump, rock or retaining walls · Tree removal, arborist or geotech reports · Fire sprinklers where the city requires them · Appliance purchase and installation · Material upgrades beyond the allowance. We price these in writing if your project needs them.
Detached DADUs, 300 to 600 sq ft
Backyard cottage
Typical size: 300 to 600 sq ft · studio or 1 bedroom, 1 bath
Common layouts: Studio cottage · 1 bedroom cottage · City pre-approved plan
Typical timeline: 5 to 8 months on site, after design and permits
Basic
Functional + affordable
Starting at
$325,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Slab or crawl space foundation
- Simple gable or shed roof
- Kitchen and 1 bath
- Siding, roofing and windows
- Insulation to Seattle Energy Code
- LVP flooring and ductless heat pump
Best for: A small home of its own in the backyard
Mid-Range
Style + durability
Starting at
$400,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Everything in Basic
- Kitchen with quartz counters
- Tiled walk-in shower
- Covered entry and larger windows
- Laundry closet
- Lighting plan
Luxury
Design + premium finish
Starting at
$500,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Vaulted ceiling and clerestory windows
- Custom cabinetry and stone counters
- Premium exterior cladding
- Heated bath floor
- Covered porch or deck
- Full selections with our designer
Super-Luxury
No limits
Starting at
$650,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Architectural roof and glass walls
- Full custom millwork
- Slab stone surfaces
- Smart lighting and controls
- Landscaped entry path
- Dedicated design and project management
Design and permits: drawings, structural engineering, survey and permit fees are priced separately in the design phase. See design and permit services. Appliance purchase and installation are not included. We prepare the connections.
Not included in starting prices: Design, structural engineering, survey and permit fees · City plan review and utility connection fees · King County sewer capacity charge · New electrical service, water meter or side sewer · Long utility trenches, sewer pump, rock or retaining walls · Tree removal, arborist or geotech reports · Fire sprinklers where the city requires them · Appliance purchase and installation · Material upgrades beyond the allowance. We price these in writing if your project needs them.
Detached DADUs, 600 to 1,000 sq ft
Two bedroom backyard home
Typical size: 600 to 1,000 sq ft (1,200 in Bellevue) · 2 bedrooms, 1 to 2 baths
Common layouts: Single story · Two story · Unit over a garage
Typical timeline: 6 to 10 months on site, after design and permits
Basic
Functional + affordable
Starting at
$425,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Foundation, framing, roof and exterior
- 2 bedrooms and 1 bath
- Kitchen with standard cabinets and quartz
- Insulation to Seattle Energy Code
- LVP flooring
- Heat pump
Best for: A full size second home on the lot
Mid-Range
Style + durability
Starting at
$500,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Everything in Basic
- Second bath or powder room
- Semi-custom kitchen
- Tiled walk-in shower
- In-unit laundry
- Covered entry
Luxury
Design + premium finish
Starting at
$625,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Two story or loft layout with custom stair
- Custom cabinetry and stone counters
- Premium windows and exterior cladding
- Heated floors in baths
- Covered porch or deck
- Full selections with our designer
Super-Luxury
No limits
Starting at
$800,000+
Includes a standard material allowance. Upgrades are priced from your selections.
- Full custom millwork throughout
- Slab stone surfaces
- Spa level primary bath
- Smart home systems
- Premium windows and glass
- Dedicated design and project management
Design and permits: drawings, structural engineering, survey and permit fees are priced separately in the design phase. See design and permit services. Appliance purchase and installation are not included. We prepare the connections.
Not included in starting prices: Design, structural engineering, survey and permit fees · City plan review and utility connection fees · King County sewer capacity charge · New electrical service, water meter or side sewer · Long utility trenches, sewer pump, rock or retaining walls · Tree removal, arborist or geotech reports · Fire sprinklers where the city requires them · Appliance purchase and installation · Material upgrades beyond the allowance. We price these in writing if your project needs them.
Get a Price for Your ADU
Book a consultation. We walk the property with you and price it in writing.
What Our Clients Say
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.
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!
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.
Read more of our Google reviews, word for word, with video reviews from finished homes.
What sets the price
The 3 Things That Control Price
Converting existing space reuses the foundation, walls and roof. A detached unit needs everything new.
More square feet, a second bedroom or a second bath add framing, plumbing and time.
Distance to sewer, water and power, plus slope and access, move the price the most.
Watch for
What Can Raise the Cost
- Distance to utilitiesLong sewer, water or power runs from the street add trenching and can need a sewer pump.
- Electrical serviceA new unit can need a larger panel or a separate meter from Seattle City Light or PSE.
- Slope, rock and accessSteep lots, rock, roots and tight side yards slow excavation and delivery.
- Fire separation and sprinklersAttached units need rated walls. Redmond requires sprinklers in detached ADUs.
- Correction lettersCity comments can mean redesign and resubmittal. Clean drawings keep this short.
- Changes after walls closeMoving plumbing or windows after inspection costs more than settling it in design.
What's Included by Tier
| Feature | Basic | Mid-Range | Luxury | Super-Luxury |
|---|---|---|---|---|
| Design, engineering and permit drawings | Separate | Separate | Separate | Separate |
| Permit filing and correction letters | Managed | Managed | Managed | Managed |
| Separate entrance | Included | Included | Included | Included |
| Kitchen | Standard | Semi-custom | Custom | Full custom |
| Counters | Quartz | Quartz | Stone | Slab stone |
| Bathroom | Tub or shower | Tiled shower | Frameless glass | Spa level |
| Flooring | LVP | LVP or wood | Wood or tile | Premium |
| Heat | Heat pump | Heat pump | Heat pump | Heat pump |
| Laundry | Hookups | In-unit | In-unit | In-unit |
| Utility connections | Priced per site | Priced per site | Priced per site | Priced per site |
"Starting at" prices reflect typical completed projects in the Greater Seattle Area and are not a quote. Final pricing depends on size, layout changes, material selections, permits and what we find behind the walls. Every project is priced in writing after a site visit.
2-year workmanship warranty
Every ADU we build carries a 2-year workmanship warranty, extendable to 5 years. Materials carry their manufacturer's warranty. See warranty details
Financing options are available for ADUs and DADUs.
Prices updated October 2026
ADU and DADU Cost Questions
How much does an ADU cost in the Greater Seattle Area?
A garage or basement conversion ADU starts at $150,000+. An attached ADU starts at $250,000+. A detached DADU starts at $325,000+ for 300 to 600 sq ft and $425,000+ for 600 to 1,000 sq ft. Design, engineering and permit fees are priced separately.
How much does a backyard cottage (DADU) cost?
A 300 to 600 sq ft DADU starts at $325,000+, and $400,000+ at Mid-Range. A 600 to 1,000 sq ft DADU with 2 bedrooms starts at $425,000+.
How much does a garage or basement conversion ADU cost?
Conversion ADUs start at $150,000+. They reuse the existing foundation, walls and roof, which is why they cost less than a new unit.
What does an ADU cost per square foot?
A recent 1,000 sq ft DADU we contracted came to about $476 per sq ft for construction. Small units cost more per foot, because the kitchen, bath and utility connections cost about the same at any size.
Are design, engineering and permit fees included?
No. Drawings, structural engineering, survey and permit fees are priced separately during the design phase. See our design and permit services page.
What is not in the starting price?
City fees, the King County sewer capacity charge, utility company charges, long utility runs, unusual site work and appliances. We price these in writing for your lot.
What is the King County sewer charge for an ADU?
In 2026 King County bills an ADU at 0.59 of a home, about $46 a month for 15 years. A discounted lump sum payment is available from the county.
Do I need a permit for an ADU in Seattle or Bellevue?
Yes. In Seattle, SDCI issues the building permit, with separate electrical, plumbing and side sewer permits. In Bellevue, permits are filed through MyBuildingPermit.com. We manage the permit process.
How long does an ADU take?
On site: a conversion takes 3 to 5 months, an attached ADU or small DADU 5 to 8 months and a 600 to 1,000 sq ft DADU 6 to 10 months. Design and permits usually take 4 to 9 months before that.
How can I keep the cost down?
Check the lot before drawings, consider a city pre-approved plan, convert existing space where you can, keep the kitchen and bath on one wet wall and settle selections before construction.
What warranty do you give?
Every ADU carries a 2-year workmanship warranty, extendable to 5 years. Materials carry their manufacturer's warranty.
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.