What Is a Granny Flat? Granny Flat vs. ADU, Explained

A granny flat is a small, self-contained second home on the same lot as a single-family house, usually with its own kitchen, bathroom, and entrance. In California, that structure has an actual legal name: an Accessory Dwelling Unit (ADU). “Granny flat” is just the informal, older name people still use in everyday conversation, real estate listings, and Google searches. It doesn’t carry any separate legal meaning of its own.

The short version: Granny flat, in-law unit, casita, and backyard cottage are all common nicknames for the same legal structure, an ADU. None of those informal terms changes the size limits, permitting process, or rules that apply. The one real exception is a JADU (Junior ADU), which genuinely is a separate legal category with its own size cap and rules. For the full picture on ADUs themselves, see our complete guide, what is an ADU.

Granny flat, in-law unit, casita, backyard cottage: all the same thing

Walk through any California neighborhood and you’ll hear a handful of different words for what is, legally, the exact same kind of structure:

• Granny flat, the oldest and most common nickname, from the idea of housing an aging grandparent nearby.
• In-law unit or mother-in-law unit, same idea, different relative.
• Casita, common in Spanish-influenced regions of California, meaning “little house.”
• Backyard cottage, a newer, more design-forward term that’s become popular in the last decade.
• Secondary unit or accessory apartment, older planning-department language that predates the modern ADU term.

All of these describe a detached or attached second home on a residential lot. None of them is a distinct permitting category. When you apply for a permit, fill out a zoning form, or talk to a contractor, the term your city, county, and state actually use is ADU. If you show up asking about a “granny flat permit,” most planning departments will simply translate that to “ADU permit” and move on, the terms are interchangeable in practice.

Where the term “granny flat” comes from

The phrase predates California’s modern ADU law by decades. Long before the state passed the ADU legislation that reshaped zoning starting in 2017, cities occasionally allowed small “granny units” or “granny flats” as a narrow exception, usually only for housing an actual elderly relative, often with strict owner-occupancy and family-relationship requirements attached.

That history is part of why the term still carries a specific mental image: a small unit built for an aging parent. But the legal reality has moved well past that original use case. Modern state ADU law removed the family-relationship requirement entirely. You can build what people still colloquially call a “granny flat” and rent it to a stranger, house an adult child, use it as a home office, or live in it yourself while renting out the main house. The nickname stuck around; the restriction it used to describe did not.

The one term that isn’t a synonym: JADU

There’s exactly one place this terminology gets genuinely confusing, and it’s worth calling out directly: a JADU (Junior Accessory Dwelling Unit) is not just another nickname for the same thing. It’s a distinct legal category with its own rules.

A JADU is capped at 500 square feet, must be carved out of the existing walls of your house rather than built as a new detached structure, is allowed an efficiency kitchen instead of a full one, and requires the property owner to live in either the main house or the JADU. A standard ADU has none of those restrictions. It can reach up to 1,200 square feet, sit fully detached in the yard, include a full kitchen, and carries no owner-occupancy requirement at all.

So while “granny flat,” “in-law unit,” “casita,” and “backyard cottage” are all interchangeable informal names for an ADU, “JADU” is a genuinely different thing with a genuinely different rulebook. We break down the full comparison, size limits, kitchens, financing, and which one fits your goal, in JADU vs. ADU.

What a “granny flat” looks like when it’s factory-built

The nickname might conjure an image of a converted shed or a cramped studio, but a modern, factory-built ADU is a real home. At Framework First, what most people would call a granny flat is one of our 14 configurations, ranging from 405 to 1,200 square feet, priced from $199,000 to $599,000 all-inclusive, built roughly 97% in our Salinas factory before crane delivery to your lot. That means real 2x6 exterior wall framing, James Hardie fiber-cement siding, mini-split HVAC, and Trex composite decking, the same construction standard whether the finished home is 450 square feet or 1,200.

Something worth knowing if you’re shopping for a “granny flat” and also weighing your options more broadly: the same 14 floor plans we build as backyard units also work as full, stand-alone primary residences. If a family member needs their own place but there isn’t an existing house on the lot to attach to, the identical model can be built as their only home rather than a backyard addition. See primary homes for how that works and which configurations are best suited to it.

Which term should you actually use?

For day-to-day conversation with family, neighbors, or on social media, use whichever term feels natural, “granny flat,” “in-law unit,” “casita,” they all communicate the same idea and nobody will correct you. But once you’re doing anything official, checking your city’s zoning code, filling out a permit application, calling a lender, or requesting a feasibility study, switch to “ADU.” That’s the term your city’s planning department, your lender’s underwriting guidelines, and California’s Government Code all use, and using it consistently avoids any confusion about which rules apply to your project.

If you’re specifically talking about a small unit built inside your existing house’s walls rather than a new detached structure, use “JADU,” since that’s the one case where the informal and formal vocabulary genuinely diverge.

Frequently asked questions

Is a granny flat the same as an ADU?

Yes. “Granny flat” is an informal, older nickname for what California law formally calls an Accessory Dwelling Unit (ADU). They describe the exact same kind of structure, a self-contained second home with its own kitchen, bathroom, and entrance, on a lot with an existing house. There’s no separate size limit or rulebook that applies specifically to something called a “granny flat.”

Do I need a special permit for a granny flat versus an ADU?

No. Since a granny flat is simply the informal name for an ADU, you apply for the same ADU permit through your city or county’s planning department. Using the term “granny flat” on an application won’t change the process, most departments will just process it as a standard ADU permit.

Can a granny flat be rented to someone who isn’t family?

Yes. California’s modern ADU law removed the requirement that an ADU (or “granny flat”) be occupied by a relative. You can rent it to anyone, use it for a family member, or live in it yourself. The one exception is a JADU, which still requires the property owner to occupy either the main house or the JADU unit.

What’s the difference between a granny flat and a casita?

Nothing legally. “Casita” and “granny flat” are both informal names for the same structure, an ADU. “Casita” is more common in Spanish-influenced regions of California and tends to describe a similar architectural style, but it doesn’t refer to a different permitting category or size limit.

How big can a granny flat be in California?

Under state ADU law, a detached unit can reach up to 1,200 square feet, and cities must allow at least 800 square feet at 16 feet in height. If what you’re picturing is smaller, closer to a converted bedroom inside the existing house, you may actually be describing a JADU, which is capped at 500 square feet. See JADU vs. ADU for the full breakdown.

Where can I read the full picture on ADUs?

Our complete guide, what is an ADU, covers size limits, cost, the permitting process, and how the 2026 law works from the ground up. This page is meant as a quick terminology check; that one is the deep dive.

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Whatever you call it, granny flat, in-law unit, or ADU, the next step is the same: find out what your lot can actually support. Start with a free feasibility study, or browse the full model lineup to see real sizes and prices.