ADU types
Granny flat: what it is and which rules apply
By the HyreADU research desk. Last checked .
The short answer
A granny flat is an informal name for an accessory dwelling unit: a small, self-contained home on the same lot as a main house, with its own kitchen, bathroom and entrance.
The name comes from housing a grandparent, but anyone can live in one. In law it is an ADU, so the ADU rules of your state and city apply.
| Other names | Where you hear it |
|---|---|
| Granny flat, granny unit | Common across the US, Australia, UK |
| In-law unit, mother-in-law suite | Common in the US |
| Backyard cottage, DADU | Seattle and the Pacific Northwest |
| Casita | The Southwest |
| Carriage house, garage apartment | Older neighborhoods, over garages |
| Accessory dwelling unit (ADU) | State and local law |
What building a legal granny flat involves
A legal granny flat needs the same things as any ADU: a building permit, a design that meets the local ADU rules, a foundation, and connections to water, sewer or septic, and power.
It passes inspections during the build and a final inspection at the end. A unit built without permits can be legalized in some cases, but that process is a legal and building question we cover only in our legalization research.
What makes a granny flat legal
A granny flat is legal when it is permitted as an ADU. In California that means meeting state ADU law (Gov. Code §§ 66310–66342) and your city’s ordinance, which together set size, height, setbacks and fees. Other states have their own laws; our ADU laws by state research covers several.
The forms a granny flat can take
- A detached cottage in the backyard. See detached ADU.
- An attached unit joined to the house.
- A converted garage.
- A unit over a garage.
- In California, a junior ADU inside the house, up to 500 sq ft.
Who lives in granny flats
Despite the name, many granny flats house tenants. In the 2021 UC Berkeley survey of California ADU owners, about half of units were generating rental income and 16 percent housed a relative or friend at no cost. See who lives in ADUs.
Where to start
Start with what your lot allows. The ADU feasibility checker lists the questions for your planning department, and can I build an ADU on my property walks through the usual blockers.
A mistake to avoid
The mistake to avoid is buying a “granny flat kit” or a “granny pod” on the assumption it can be placed without a permit. If someone will live in it, it is a dwelling, and dwellings need permits. Ask your building department before you buy.
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 granny flat the same as an ADU?
Yes, in everyday speech. The legal term is accessory dwelling unit.
Does a granny flat need a permit?
Yes, as an ADU.
Can anyone live in a granny flat?
Yes. Most state ADU laws do not limit who may live there.
How big can a granny flat be?
It depends on your state and city. See ADU size.
Sources
- Gov. Code § 66313 (ADU and junior ADU definitions). Checked October 7, 2026.
- Gov. Code § 66321 (size and height standards a city may not impose). Checked September 5, 2026.
- Chapple, Ganetsos and Lopez, Implementing the Backyard Revolution, UC Berkeley Center for Community Innovation (2021). Checked September 5, 2026.
- Seattle SDCI, Accessory Dwelling Unit (backyard cottage). Checked October 27, 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.