ADU and DADU, Seattle and Bellevue
ADU vs DADU in Seattle: what is the difference?
An ADU, an accessory dwelling unit, is a secondary living space attached to or inside your existing home, such as a basement or garage conversion. A DADU, a detached accessory dwelling unit, is a separate standalone structure in the backyard with its own entrance, kitchen and living space.
Both sit on the same lot as the main house and both add legal, livable square footage. The choice between them is decided by the lot, the existing space and the local zoning rules.
What is an ADU?
An ADU, an accessory dwelling unit, is a secondary housing unit on a single-family residential lot. It can be attached to the main home, built above a garage, or converted from existing space such as a basement. It sits on the same lot as the primary house.
These units usually contain all the basic amenities for living, including a kitchen, a bathroom and a bedroom. Other common names for them are granny flat, in-law unit and backyard cottage.
What is a DADU?
A DADU, a detached accessory dwelling unit, is a fully independent structure built separately in the backyard, with its own entrance, kitchen and living space. It is a building in its own right rather than an extension of the house, which is why most small cottages and tiny houses fall into this category.
Because it stands alone, a DADU is the more isolated and private of the two, and it provides independence for whoever uses it.
So what is the actual difference?
One word: attachment. An ADU shares the building with your home, whether that is a conversion inside it, a space above the garage or an addition on the side or the back. A DADU is a separate structure standing on its own footprint in the yard. Both sit on the same lot as the main house, and both add legal, livable square footage.
- ADU: attached to or inside the existing home. Basement conversion, garage conversion, space above a garage, or an addition connected to the house.
- DADU: a standalone building in the backyard, with its own entrance, kitchen and living space.
Which one costs more to build?
A detached unit generally costs more than an attached one. An attached ADU shares infrastructure with the main house, walls and facilities that are already in place, which keeps construction costs relatively lower. A DADU is new construction from the ground up, so the foundation, the envelope and the utility connections are all its own.
Utility runs matter more than people expect on a detached build: connecting water, electricity and sewer costs more the further the unit sits from those connections. Size drives the rest, because a larger unit demands more materials and more labor.
For published figures, see how much it costs to build an ADU and the Seattle remodel cost page, which carries the detached ADU and DADU planning range from the homepage table.
What do people build them for?
To add living space, to house a family member, or to increase property value. The uses named across the site are more living space, a home office and housing for family members, which is why an ADU or a DADU turns up so often on lots where the main house has run out of room.
Do you need a permit for either one?
Yes. Both are regulated construction, and most remodels that change plumbing, electrical, structure or layout require a building permit from your local jurisdiction.
Zoning matters as much as the building code here. Each municipality has its own requirements for unit size, placement and features such as parking and entrance pathways, and designing to those rules from the start avoids costly revisions and delays. See do you need a permit to remodel in Seattle.
Volcano Builders handles everything from permit applications to final construction, and is responsible for the state permissions during the build.
Which one suits your lot?
If the space already exists inside the house, an ADU is usually the shorter route: a basement conversion or a garage conversion uses the structure that is already there without expanding the footprint of the property. A basement conversion normally needs waterproofing, egress windows and insulation to meet building codes and make the space comfortable.
If the yard has room and privacy is the point, a DADU gives a fully independent building. Because these units are small, the plan is what makes them work, so optimizing the use of space is the whole design problem.
Volcano Builders is a licensed ADU contractor in Seattle with 20+ years of experience building custom ADUs and DADUs across King, Snohomish and the surrounding counties in Washington State. The consultation is by appointment, and you get a quick estimate for smaller projects or a detailed proposal for more complex ventures within 7 business days.
Frequently Asked Questions
What is the difference between an ADU and a DADU?
An ADU, an accessory dwelling unit, is a secondary living space attached to or inside your existing home, such as a basement or garage conversion. A DADU, a detached accessory dwelling unit, is a separate standalone structure, like a backyard cottage. Both add legal, livable square footage on the same lot as the main house.
Does a DADU have its own kitchen and bathroom?
Yes. A DADU is a fully independent structure built separately in the backyard, with its own entrance, kitchen and living space, and these units commonly contain all the basic amenities for living, including a kitchen, a bathroom and a bedroom.
Is a garage conversion an ADU?
Yes. A garage conversion is one of the standard forms of an ADU. It uses existing space without expanding the square footage of the property.
Do you need a permit to build an ADU or a DADU in Seattle?
Yes. Both are regulated construction. Each municipality also has its own zoning requirements for unit size, placement and features such as parking and entrance pathways, so the design has to comply from the start to avoid costly revisions and delays.
