ADU comparisons
ADU vs DADU: what the two terms mean
By the HyreADU research desk. Last checked .
The short answer
ADU vs DADU is mostly a naming difference. ADU means accessory dwelling unit, the umbrella term.
DADU means detached accessory dwelling unit, a separate backyard home, and is common in Seattle and the Pacific Northwest.
In those codes an attached unit inside or joined to the house may be called an AADU. California law uses “detached ADU” instead.
| Term | Meaning | Where you see it |
|---|---|---|
| ADU | Any accessory dwelling unit: attached, detached or converted | National; state laws |
| DADU | Detached accessory dwelling unit, often called a backyard cottage | Seattle and other Northwest cities |
| AADU | Attached accessory dwelling unit | Seattle code |
| Detached ADU | Same idea as a DADU | California Gov. Code §§ 66321, 66323 |
| JADU | Junior ADU inside a house, up to 500 sq ft | California only |
Why the term matters
The term matters because your city’s code uses one of them, and the rules sit under that heading. Searching a Seattle code for “detached ADU” may miss the DADU section. Seattle’s Department of Construction and Inspections uses both AADU and DADU on its ADU page.
What a DADU usually involves
A DADU is a new building, so it needs its own foundation, roof and utility runs. That is the same work as any detached ADU. Builders who do this work locally may call it a backyard cottage.
Which one to choose
Choosing between a DADU and an attached unit is the real decision. Our attached vs detached ADU page compares privacy, height and utilities.
A mistake to avoid
The mistake to avoid is reading another state’s rules as yours. A blog post about DADUs in Seattle will quote height, size and parking numbers from the Seattle code that do not apply in Portland, Denver or Los Angeles.
Look up your own city’s code under whichever term it uses, and note the date on the page you read, because Northwest cities have changed these rules often.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Is a DADU the same as a granny flat?
Often in everyday speech. “Granny flat” is an informal term for any ADU. See granny flat.
Does California use the term DADU?
Not in the statute. California law says detached accessory dwelling unit.
Can a lot have a DADU and an AADU?
Some cities, including Seattle, allow more than one ADU on a lot. Check the current local code.
Is a DADU a tiny house?
It can be small, but it is a permitted dwelling on a foundation. See ADU vs tiny home.
Sources
- Seattle SDCI, Accessory Dwelling Unit (backyard cottage). Checked October 27, 2026.
- Gov. Code § 66321 (size and height standards a city may not impose). Checked September 5, 2026.
- Gov. Code § 66323 (ADUs a city must approve ministerially). Checked September 5, 2026.
- Gov. Code § 66313 (ADU and junior ADU definitions). Checked October 7, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.