Most states do not have their own statewide legal definition of “ADU.”
The usual pattern is the opposite of California’s.
How it actually works
There is no federal definition of an accessory dwelling unit. The phrase is planning jargon that localities and a minority of states have written into law.
As of 2026, roughly
18–20 states have a statewide ADU statute that requires (or strongly encourages) cities to allow them. Those statutes almost always include a definition. Examples: California, Oregon, Washington, Massachusetts, New Hampshire, Arizona, Colorado, Maine, Montana, Vermont.
The other
30-plus states — including Tennessee, Texas, Florida, Georgia, Illinois, Pennsylvania, and most of the South and Midwest — leave ADUs to
city and county zoning. There is no state definition. Each locality that wants the concept writes its own, or never uses the term at all and instead regulates “guest houses,” “garage apartments,” “accessory apartments,” or “second dwelling units.”
So it is not “each state has its own definition.” It is:
- a minority of states define the term in statute and preempt local bans;
- most states have no state definition;
- thousands of cities and counties each have (or lack) their own.
The definitions that do exist are close
Where a definition exists, it is usually the same functional idea:
a second dwelling on the same lot as a principal house, with its own sleeping, cooking, and sanitation.
California is unusual in also creating a separate statutory category (
JADU) and in how tightly the state definition binds every city. New Hampshire’s statute, for example, is closer to the common core: a living unit on a lot with a single-family dwelling that has independent sleeping, eating, cooking, and sanitation. Nashville’s local “DADU” definition adds extra tests (must be detached and subordinate in size/height/purpose) that California’s state definition does not require.
Bottom line
“ADU” is a shared planning term, not a uniform legal term. California is on the high-definition, high-preemption end. Tennessee is on the local-control end. Most states look more like Tennessee than California.
