What Does DADU Mean?
DADU stands for detached accessory dwelling unit. It's a second, smaller home built on the same lot as your main house, but standing on its own, with its own entrance, kitchen, and bathroom. People also call it a backyard cottage, a granny flat, or a mother-in-law unit, but DADU is the term you'll see in city planning documents and zoning code around Seattle and King County.
DADU vs. ADU vs. AADU
"ADU" (accessory dwelling unit) is the umbrella term. It covers any second housing unit on a residential lot. Under that umbrella, cities split ADUs into two types:
- AADU (attached accessory dwelling unit): built inside or attached to the main house. A basement apartment, or a unit added over the garage that shares a wall with the house.
- DADU (detached accessory dwelling unit): a separate structure. New construction in the backyard, or a garage converted into living space.
If someone tells you they're building a "DADU," they mean the standalone version, not a basement conversion. The distinction isn't just vocabulary. Seattle and most surrounding cities apply different rules to attached and detached units, so which one you're building changes what you're allowed to do.
Why the label affects your project
Local code sets separate limits for DADUs on things like height, roof pitch, distance from property lines, and total square footage, and those numbers are not the same from city to city. Seattle, Bellevue, Kirkland, Issaquah, Newcastle, and SeaTac each write their own zoning code, so a DADU that's allowed on a given lot in one city might need a variance, or simply isn't sized the same way, a few miles away in another. Before you plan anything, the first real question is which jurisdiction your property sits in and what that city's code allows on your specific lot, not what a neighbor built or what you read online for a different city.
A few other things the "detached" status usually affects:
- Utilities. A DADU is more likely to need its own water, sewer, and power connections worked out during design, since it isn't sharing walls or systems with the main house the way an AADU can.
- Site work. Because it's a new freestanding structure, a DADU triggers its own foundation, drainage, and grading work, separate from anything happening to the primary house.
- Use. Both types can be rented out or used for family, but a fully detached unit gives tenants or family members more separation and privacy from the main household, which is often the actual reason people choose DADU over AADU in the first place.
What actually determines if you can build one
Two things decide it before anything else: your zone (most single-family and many multifamily zones in the area now allow ADUs, but confirm yours) and your lot size and shape, since setbacks and coverage limits determine how much of the backyard is actually buildable. After that, the details come down to lot coverage math, tree retention rules if you have significant trees, and how much of the backyard is left once setbacks are drawn in. This is worth checking early, because it can change a project from "backyard cottage" to "second-floor addition over the garage" before you've spent time on a design that won't fit.
Getting from "what does this mean" to a real plan
Knowing the term is the easy part. The harder part is translating your specific lot, your city's code, and what you actually want the unit for (rental income, aging parents, a home office) into a design that gets permitted the first time. That's the stage where a local builder who works in this zoning code regularly, rather than one seeing it for the first time, saves you months. Love Construction builds ADUs and DADUs across Seattle, Bellevue, Kirkland, Issaquah, Newcastle, and SeaTac, and can tell you early, before you've committed to a design, what your lot will actually support.