Planning guide

DADU vs ADU: The Difference, and Which One Your Property Can Actually Have

ADU stands for accessory dwelling unit. It's the umbrella term for any secondary, independently livable home on a single-family lot, one with its own kitchen, bathroom, sleeping area, and entrance. DADU stands for detached accessory dwelling unit. It's one type of ADU: a freestanding structure with its own foundation and roof, built apart from the main house. Every DADU is an ADU. Not every ADU is a DADU.

In everyday conversation, people use "ADU" loosely to mean an attached unit, like a basement apartment or a garage conversion, and "DADU" specifically for a standalone backyard cottage. Seattle's building code and most King County cities use the fuller terms: attached ADU and detached ADU (DADU). If a planner or builder asks which one you're building, they're asking about physical connection to your house, not the size or use of the unit.

What actually changes between attached and detached

The attached-vs-detached line isn't just terminology. It changes what you can build, how long it takes, and what it costs to get there.

Site work. An attached ADU, most often a basement or garage conversion, uses walls, a foundation, and a roof that already exist. A DADU needs all three built from scratch: new foundation, new framing, a full roof structure. That's the single biggest reason a DADU generally costs more and takes longer than converting existing space.

Utilities. A DADU typically needs its own water, sewer, and electrical connections run out to the backyard structure. An attached conversion can often tie into the systems already serving the house, though older homes sometimes need a panel upgrade either way.

Permitting. Both types need a building permit in Seattle and the surrounding cities. A DADU adds a site plan showing setbacks from your property lines, structural plans for the new foundation, and sometimes design review for how the exterior reads from the street or alley. An attached conversion usually has a more direct path because you're working inside an existing envelope.

Privacy and use. A DADU gives real separation. No shared walls, often a separate address, its own outdoor space. That matters if the unit is for a renter, an adult child, or aging parents who want to be close but not under the same roof. An attached ADU works well when the goal is convenience or budget, and full separation isn't the priority.

Washington changed the rules in 2023, and it matters here

Washington State passed a law in 2023 (HB 1337) that overrode a lot of the old city-by-city restrictions on ADUs. As of 2026, in most Seattle-area cities:

• Owner-occupancy is no longer required. You can build a DADU or an attached

ADU as a straight rental property without living on the lot yourself.

• Cities generally can't require extra parking as a condition of ADU

approval.

• Most lots that can fit single-family zoning are now required to allow two

ADUs, not one. In practice, that means a homeowner with a suitable lot can build an attached ADU and a detached DADU on the same property at the same time, something that was often blocked outright under the older code.

Local setback, lot-coverage, and design-review rules still apply, and they're specific to your zone and lot shape, so the state law change doesn't mean anything goes. But it opens an option that didn't exist a few years ago: two income-producing units on one lot instead of choosing between them.

Which one fits your property

A few questions tend to settle it fast:

• Is there existing space to convert? A basement with decent ceiling

height, or an attached garage, points toward an attached ADU. It's the faster, lower-cost path when the bones are already there.

• Is your rear yard big enough, with the right setbacks? A DADU needs

room to sit apart from both the house and the property line. If your lot has usable backyard depth, a DADU is worth pricing out even if it costs more, because it opens the door to a second unit later under the new two-ADU rule.

• Who's going to live there, and how separate does it need to be? Full

privacy for a long-term tenant or a family member who wants their own space favors a DADU. A short-term or lower-budget rental, or space for visiting family a few times a year, often doesn't need that separation.

• Do you want both, eventually? If your lot can support it, building the

attached unit first and planning the DADU as a second phase is a reasonable way to spread out the cost while still ending up with two units.

None of these questions has one right answer independent of your actual lot. A site visit and a review of your specific zoning and setbacks is the only way to know for certain what's buildable.

Where this fits with a real project

If you're at the point of figuring out whether your lot supports an attached ADU, a DADU, or both, that's the conversation to have with a builder who handles the permitting and design work locally rather than guessing from a national cost calculator. Love Construction works on ADU and DADU projects across Seattle and the surrounding King County cities, and is a Passive House certified builder for homeowners who want the added unit built to a tighter energy standard from the start.

The short version: ADU is the category, DADU is the detached kind, and which one makes sense for you comes down to what your lot can physically hold and how separate you need the space to be.