ADU, Granny Flat, Casita, In-Law Suite: What Every Term Actually Means

The vocabulary problem is real and it costs people time. A homeowner asks their city about building a “guest house” and gets routed toward a discretionary permit process. The same homeowner asking about an ADU gets a ministerial approval that the city can’t easily refuse.

Same building. Different word. Different outcome.

Every term, side by side

What each term means, and what it maps to legally

Term	What people mean	Legal equivalent in CA	Where it's used
Accessory dwelling unit (ADU)	The official category	ADU, the statutory term	California law, planning departments
Junior ADU (JADU)	Small unit inside the house	JADU, a distinct legal category	California law
Granny flat	Small home for an aging parent	ADU	Australia; older American homeowners
Mother-in-law suite	Space for a relative	ADU, or JADU if inside the house	Eastern and Midwestern US
In-law suite	Same as above	ADU or JADU	Real estate listings nationwide
Casita	Little house	ADU	Southwest US, especially AZ, NM, TX
Backyard cottage	Detached small home	ADU	Pacific Northwest, Seattle ordinances
Guest house	Detached space for visitors	ADU only if it has a kitchen and bath	Nationwide, informal
Secondary suite	Second unit on a lot	ADU	Canada; some US planning documents
Carriage house	Unit above a garage	ADU	Older Eastern and Midwestern cities
Second unit	Pre-2017 California term	ADU	Older California ordinances

The only distinction that carries legal weight

Two categories exist in California law, and the difference between them is worth understanding.

Accessory dwelling unit (ADU). A complete independent dwelling on a lot with a primary residence, with its own kitchen, bathroom, and entrance. Can be detached, attached, or converted from existing space. Your city cannot impose owner-occupancy requirements. Size caps must allow at least 850 square feet, or 1,000 with more than one bedroom.

Junior accessory dwelling unit (JADU). No more than 500 square feet, carved out of the existing single-family residence. Needs only an “efficiency kitchen,” meaning appliances, a food prep counter, and storage cabinets. May share a bathroom with the main house. Owner-occupancy can be required, but only if the JADU shares sanitation facilities with the primary home.

One useful quirk: for JADU purposes, an attached garage counts as part of the single-family dwelling. So converting an attached garage into a 500-square-foot unit inside the home’s footprint can be done as a JADU.

Where the line falls: kitchen and bathroom

The question that determines everything is not what you call the structure. It’s whether it contains a kitchen and a bathroom.

• No kitchen, no bathroom means an accessory structure: a shed, a studio, a pool house, a home office. Not a dwelling. Different, generally simpler rules, and none of the ADU protections.
• Kitchen and bathroom means a dwelling unit. In California, an ADU, with all the state protections and all the requirements.

This is why “guest house” is the most slippery term on the list. A guest house with a bathroom and a hot plate is arguably a dwelling. A guest house with a full kitchen definitely is. People build the first, call it the second, and discover the distinction during a sale or an insurance claim.

Why the legal term is the one that matters

Calling it an ADU when you talk to your city invokes a body of state law that has spent several sessions systematically removing local discretion. As of 2026, your city cannot:

• Require you to live on the property (ADUs, under § 66315)
• Impose side or rear setbacks greater than four feet on a new detached unit
• Cap your size below 850 square feet, or 1,000 with more than one bedroom
• Use open space or lot coverage rules to deny an 800-square-foot unit with four-foot setbacks
• Require replacement parking when you convert or demolish a garage to build one
• Take longer than 15 business days to tell you whether your application is complete, or 60 days to approve or deny it

None of that attaches to the phrase “guest house.”

Regional notes

Granny flat is Australian in origin and is the standard term there. In the US it’s most common among older homeowners, and the search intent behind it skews strongly toward building for an aging parent, which is why we wrote that version separately.

Casita is Spanish for “little house” and is common in Southwest architecture, where it often describes a detached guest room off a courtyard rather than a full independent unit. It’s also, confusingly, a travel trailer brand and a very common restaurant name. See what a casita is.

Carriage house originally meant exactly that: the building where the carriage lived, with quarters above for whoever drove it. The modern usage, a unit above a garage, is a direct descendant.

Second unit was California’s own term before the state standardized on “accessory dwelling unit.” You’ll still find it in older municipal codes, which is a reasonable signal that the ordinance predates the current statute and may not be compliant with it.

The one-sentence version

If it has a kitchen and a bathroom and it’s on a lot with a house, it’s an ADU, whatever your grandmother calls it and whatever the listing says.

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Sources

• California HCD, Accessory Dwelling Unit Handbook
• Best Best & Krieger, SB 477 relocated state ADU law to a new Government Code chapter
• Best Best & Krieger, Governor Newsom Signs Four New ADU Bills