Granny Flat, Casita, In-Law Suite: Are They the Same as an ADU? | 1-800-ADU-Pros
Granny flat, casita, in-law suite, mother-in-law suite, guest house, backyard cottage — are they the same as an ADU? Mostly yes. Here's where the terms diverge in LA.
Written by 1-800-ADU-Pros
Granny Flat, Casita, In-Law Suite: Are They the Same as an ADU?
Mostly, yes. Granny flat, casita, in-law suite, backyard cottage, guest house — they're all everyday names for what California law calls an ADU. Here's where the terms line up, where they don't, and what actually makes it legal in Los Angeles.
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If you've searched "build a house for my mom in my backyard" or "how much is a granny flat in LA," you've already pictured the thing you want. You just may not know it has a clear legal name — and a defined permit pathway in California. That gap is bigger than most people think: a Freddie Mac survey found that 71% of Americans are unfamiliar with the term "ADU", yet once it's explained, 32% of non-owners say they'd be interested. Most homeowners reach for a synonym first.
The 30-second answer
Yes — in everyday use, granny flat, casita, in-law suite, mother-in-law suite, backyard cottage, guest house, and garage apartment all describe the same thing California law calls an Accessory Dwelling Unit (ADU). They're informal nicknames; "ADU" is the legal term that unlocks the streamlined state permit rules. There's one real catch: an in-law or mother-in-law "suite" with no kitchen that just shares the main house isn't a legal ADU at all — it's a bedroom. The kitchen plus independent living is what turns a spare room into an ADU.
So the names aren't wrong — they're just imprecise. Below, we map each common term to what it usually means and whether it counts as a legal ADU in Los Angeles, then explain the one feature that decides it.
Same backyard cottage, seven different names
Here's the quick translation table. The deciding question in the right column is always the same: does it have its own kitchen, bathroom, sleeping area, and entrance?
| Common name | What people usually mean | Is it a legal ADU? |
|---|---|---|
| Granny flat | A small standalone or attached unit, often for an aging parent | Yes — if it has a kitchen + entrance |
| Casita | A small detached "little house" in the yard | Yes — if it has full independent living |
| Backyard cottage | A detached unit in the rear yard | Yes — this is a classic detached ADU |
| In-law / mother-in-law suite | A room or wing for a relative, often sharing the main home | Only if it has its own kitchen + entrance |
| Guest house | A detached space for visitors | Sometimes — many older ones are unpermitted |
| Garage apartment | Living space converted from or built over a garage | Yes — a permitted garage conversion ADU |
| Junior ADU (JADU) | A unit carved out of the existing house | Yes — a JADU, with its own rules (below) |
The names overlap on purpose — people describe what they can picture. The legal status comes down to one thing, which we get into next. If you're starting from zero, our plain-English guide to what an ADU is covers the basics.
What actually makes it an ADU (vs. just a fancy room)
California's housing department (HCD) is clear on the definition. An ADU is a self-contained residence with its own:
- Kitchen — permanent cooking facilities, not just a microwave on a shelf
- Bathroom — full bath with its own plumbing
- Sleeping area — space to live, not just visit
- Independent entrance — someone can come and go without walking through the main house
That's the dividing line. A "mother-in-law suite" that's really a bedroom and bath off the hallway — no kitchen, shared front door — is a great setup for a relative, but it is not a legal ADU. It can't be rented as a separate unit, doesn't get its own address, and won't show up on title as a second dwelling. Add a kitchen and a private entrance and the same space becomes an ADU.
What about a JADU?
A Junior ADU (JADU) is the in-between option: up to 500 sq ft carved out of the existing house (often a converted bedroom or attached garage) with an efficiency kitchen. It can share a bathroom with the main home, but owner-occupancy is required — you have to live in the house or the JADU. It's the cheapest way to add a legal unit because you're working inside the existing footprint. See our breakdown of ADU vs. JADU for which one fits your lot and budget.
"Guest house" is the trickiest word
A lot of LA properties have an existing "guest house" that was built decades ago without permits, or permitted as something else (a "rec room," a "studio"). On paper it might not be a legal dwelling at all. If that's your situation, you don't necessarily have to tear it down — California has a path to legalize an unpermitted ADU, and bringing it up to code is often cheaper than building new.
Why so many people build one for family
The most common name — "granny flat" — tells you why this category exists. ADUs are overwhelmingly a family-housing solution: 61% of ADUs are built for multigenerational housing. And AARP found that 77% of older adults want to age in their own home, and 62% of adults 50+ would build an ADU for a loved one who needs care.
The math is part of it. Assisted living in the LA area runs roughly $4,000–$10,000 a month, every month, with nothing to show for it at the end. A granny flat is a one-time build that keeps a parent close, gives everyone privacy, and stays an asset on your property afterward — rentable later, or a place for an adult kid, an office, or a future buyer's bonus. It's not a small decision, but it's a different kind of spend than a monthly facility bill.
You're also not alone in considering it: 66% of California homeowners have considered adding an ADU.
What it costs — and what LA allows
Whatever you call it, the build is the same. In Los Angeles, ADUs run roughly $250–$400 per square foot, or $150K–$400K+ all-in depending on size and finishes. A garage-conversion granny flat is the budget end, around $100K–$225K in LA. The median California ADU comes in near $150K. We break the line items down in our LA ADU cost guide.
On size and feasibility, the rules are friendlier than most people expect. There's no minimum lot size — the state guarantees you can build at least an 800 sq ft / 16 ft unit on a qualifying lot regardless of local limits, and the City of LA allows detached ADUs up to 1,200 sq ft. So whether you wanted a tiny casita or a full two-bedroom granny flat, there's usually room. The real question is what your specific address allows — that's what the free check below answers. (For the full picture, see can I build an ADU in Los Angeles, and our overview of the types of ADUs in California.)
Not sure if your "granny flat" idea is even allowed?
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The bottom line on the names
Don't let the vocabulary slow you down. If you want a small living space on your property for a parent, a kid, a renter, or yourself, you want an ADU — whatever word you used to describe it. The one thing to get right is the kitchen-plus-entrance test: that's what separates a real, legal, valuable second unit from a glorified spare room.
Once you know that, the next move is checking what your lot can actually support, then matching with a vetted, CSLB-licensed builder. We can do both. Start with the free property check, or browse our directory of the best ADU builders in Los Angeles.
Granny flat, casita & in-law suite questions
Is a granny flat the same as an ADU?
Yes. "Granny flat" is just a common, informal name for an Accessory Dwelling Unit (ADU). As long as it has its own kitchen, bathroom, sleeping area, and independent entrance, a granny flat is a legal ADU under California law and follows the same streamlined permit rules.
Is an in-law suite a legal ADU?
Only if it has its own kitchen and a separate entrance. A typical in-law or mother-in-law suite that is a bedroom and bathroom sharing the main home's kitchen and front door is not a legal ADU — it's an extra room. Add a permanent kitchen and an independent entrance and it becomes an ADU.
What is the difference between a casita and an ADU?
There isn't a legal difference. "Casita" is Spanish for "little house" and is a common name for a small detached unit in the backyard. If that casita has full independent living — kitchen, bathroom, sleeping area, and its own entrance — it is simply a detached ADU.
What makes something a legal ADU in California?
California's housing department defines an ADU as a self-contained residence with its own permanent kitchen, a bathroom, a sleeping area, and an independent entrance so a person can live there without going through the main house. That combination — independent living plus its own entrance — is what makes it an ADU rather than just a room.
Is a guest house an ADU?
Sometimes. A guest house with a full kitchen, bathroom, and its own entrance can qualify as an ADU. But many older guest houses in Los Angeles were built without permits or permitted as a rec room or studio, so they may not be legal dwellings. California has a path to legalize an unpermitted unit, which is often cheaper than building new.
What is a JADU and how is it different from a granny flat?
A Junior ADU (JADU) is up to 500 square feet carved out of the existing house — often a converted bedroom or attached garage — with an efficiency kitchen. It can share a bathroom with the main home, but owner-occupancy is required. A granny flat is usually a larger, fully independent ADU, while a JADU is the smaller, lower-cost option built inside the existing footprint.
