Definitions
Granny Flat vs ADU vs JADU: What's the Difference in California?
Granny flat, in-law unit, ADU, JADU, casita, they're often the same thing and sometimes not. Here's exactly what each term means under California law in 2026.
In California these terms get used interchangeably, but they aren't all the same thing under the law. Here's the actual breakdown homeowners ask us about every week.
ADU (Accessory Dwelling Unit)
The legal name in California. A self-contained second living unit on a single-family or multi-family lot with its own kitchen, bathroom, sleeping area, and entrance. Can be detached, attached, or a conversion of an existing structure (garage, basement, accessory building). State law allows up to 1,200 sq ft on most lots.
JADU (Junior ADU)
A smaller carve-out inside the existing footprint of the main house. Capped at 500 sq ft. Can share a bathroom with the main house. Requires an owner to live on the property. Cheaper and faster to build than a full ADU because it doesn't need its own foundation or full utility hookups.
Granny flat / In-law unit / Casita
Informal names for the same thing, almost always a detached ADU. "Granny flat" comes from Australian/UK usage, "in-law unit" is the older Bay Area term, "casita" is more common in Southern California. None of these are legal categories. When you apply for a permit, all of them get filed as ADUs.
Guest house
Not the same as an ADU. A guest house has no kitchen and no separate utility meter, it's an accessory building you can't legally rent or live in full-time. Most homeowners who say "guest house" actually want an ADU. Building a true ADU costs only modestly more and allows rental income, multi-generational living, and resale value a guest house can't.
Manufactured / mobile home
Built to HUD code, not California building code. Cities can require these as ADUs but most homeowners pick site-built or modular instead, better finish, longer life, and stronger appraisal.
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