If you've been researching backyard homes in the Phoenix area, you've probably encountered at least four different names for the same thing: casita, ADU, granny flat, guest house, in-law suite, accessory dwelling unit. It's enough to make your head spin — especially when you're trying to figure out what's allowed on your lot.
Here's the truth: in Arizona, these terms all refer to the same structure. The differences are mostly about who's using the word and why. Once you understand that, the whole conversation becomes a lot simpler.
The Official Term: Accessory Dwelling Unit (ADU)
ADU — accessory dwelling unit — is the legal planning term used in Arizona zoning codes, state statutes, building permits, and city ordinances. When a city planner, building department, or attorney talks about a secondary backyard home, they say ADU.
By definition, an ADU is a complete, self-contained dwelling unit on the same lot as a primary residence. It has its own entrance, its own kitchen, its own bathroom, and its own sleeping area. It functions as an independent home — just a smaller one, attached to or detached from the main house on the same property.
Under Arizona's HB 2720 (effective January 2025), every city in Arizona with a population over 75,000 — including Phoenix, Scottsdale, Mesa, Tempe, Chandler, and Peoria — must allow at least one detached ADU on any single-family lot. That's the law that opened the backyard home market across the Valley.
The Arizona Term: Casita
Casita is Spanish for "little house," and it's the word most Phoenix and Scottsdale homeowners use in everyday conversation. It carries a specific Southwest architectural feel — stucco, tile, adobe influences — but in practice, people use "casita" to describe any backyard dwelling unit regardless of style.
When someone says "I want to build a casita in my backyard," they mean exactly the same thing as "I want to build an ADU on my lot." The word is cultural and regional, not legal. Your permit will say ADU. Your neighbors will say casita.
At Valley Casita Co., we use both terms — casita in conversation because it's how Arizona homeowners actually talk, ADU in permitting and legal documents because that's what the city requires.
The Informal Terms: Granny Flat, Guest House, In-Law Suite
These terms describe the same structure through the lens of how it's used, not what it is legally.
Granny Flat
A granny flat is an ADU or casita used — or originally intended — to house an elderly family member nearby. The term is common in Australia and the UK and has carried over into American usage. In Phoenix, you'll hear it most often from homeowners who are thinking about their parents or in-laws. It implies a smaller, self-contained unit with accessibility features.
Legally, a granny flat is an ADU. The zoning code doesn't distinguish between a granny flat and a casita used for rental income — they're the same structure under the same permit process.
Guest House
A guest house is typically a detached backyard structure used for visitors. The term sometimes implies a unit without a full kitchen — just a sleeping area and bathroom. In older Phoenix neighborhoods, you'll find guest houses that predate the ADU terminology and were built without kitchen hookups.
Today, most new construction guest houses are fully equipped ADUs. If you're building new, you want a full kitchen — it preserves flexibility. A unit with a full kitchen can be a guest house today, a rental tomorrow, and a multigenerational suite next year. A unit without a kitchen limits your options permanently.
Mother-in-Law Suite / In-Law Suite
A mother-in-law suite describes an ADU used specifically for a family member — usually an aging parent. It can be detached (a separate backyard structure, which is what Valley Casita Co. builds) or attached (a suite added onto the primary home or converted from a garage).
The term is about relationship and use, not structure or legal category. A mother-in-law suite that's a separate backyard building is a detached ADU. A mother-in-law suite that's a converted attached garage is an attached ADU. Both require the same permit process.
The One Real Distinction: Detached vs. Attached
While casita, ADU, granny flat, guest house, and in-law suite are all essentially synonymous, there is one meaningful structural distinction worth understanding: detached vs. attached.
Detached ADU (what Valley Casita Co. builds)
A completely separate structure in your backyard — its own foundation, walls, roof, and entrance, with no shared walls with the primary home. Maximum privacy for both households. Can be placed anywhere on the lot that meets setback requirements. Adds the most appraised value to the property.
Attached ADU
Shares at least one wall with the primary home — typically a converted garage, a basement conversion, or a room addition with a separate entrance. Lower construction cost in some cases, but less privacy and less flexibility in placement.
Valley Casita Co. specializes in detached, site-built casitas. All five of our models — from The Studio (400 SF) to The Sonoran (1,400 SF) — are designed as fully detached structures built on-site to your lot's specific dimensions.
Quick Reference: Every Term Defined
| Term | What It Means | Who Uses It |
|---|---|---|
| ADU | Accessory dwelling unit — the official legal term for any secondary dwelling on a single-family lot | City planners, building departments, attorneys, permits |
| Casita | Arizona/Southwest term for a detached backyard home — same as an ADU | Homeowners, builders, real estate agents in the Phoenix Valley |
| Granny flat | Informal term for an ADU used to house an elderly family member | Homeowners researching multigenerational options |
| Guest house | A detached backyard structure used for visitors — may or may not have a full kitchen | Homeowners, real estate listings |
| Mother-in-law suite | An ADU (detached or attached) used to house a family member — describes use, not structure | Families planning multigenerational living |
| In-law suite | Same as mother-in-law suite | Same as above |
| Accessory dwelling unit | Same as ADU — the full, unabbreviated legal term | Formal documents, state statutes, zoning codes |
Which Term Should You Use in Phoenix?
In conversation with neighbors, family, and contractors: say casita. It's how Phoenix talks, and everyone will know what you mean.
In conversations with your city's building department, your HOA, or your attorney: say ADU. That's the term your permit application will use, and it's what the city's zoning code references.
When searching Google for builders, costs, and permitting: use both. Searches for "casita builder phoenix," "ADU builder phoenix," "granny flat builder phoenix," and "guest house builder phoenix" all surface the same category of contractors. The terminology varies — the product is the same.
Does It Matter Which Term You Use for Your HOA?
This is actually important. Some HOA CC&Rs prohibit "detached structures," "secondary dwellings," or "guest houses" without specifically mentioning ADUs or casitas. If your HOA documents use different terminology, the restriction may still apply regardless of what you call the structure.
Under HB 2720, Arizona HOAs cannot prohibit ADUs that are permitted and compliant with state law — but HOAs established before January 1, 2015 with existing restrictions may still have some enforcement rights. The specifics depend on your HOA's exact CC&R language.
Before you decide what to build, run our free HOA eligibility check. We'll tell you whether your lot is in an HOA-governed area and what restrictions typically apply — in two minutes, at no cost.
Frequently Asked Questions
What is the difference between a casita and an ADU?
There is no difference. ADU (accessory dwelling unit) is the legal term used in permits and zoning codes. Casita is the everyday Arizona term for the same structure — a detached backyard home with its own entrance, kitchen, and bathroom.
What is a granny flat in Arizona?
A granny flat is an informal term for an ADU or casita used to house an elderly family member. Legally it's identical to any other ADU — same permit process, same zoning rules, same construction requirements.
What is an accessory dwelling unit?
The official planning term for a secondary dwelling on a single-family residential lot. In Arizona, ADUs are regulated under HB 2720 (effective January 2025), which requires cities with populations over 75,000 to allow at least one detached ADU on any single-family lot.
Is a guest house the same as an ADU?
Essentially yes. The main distinction: a guest house sometimes refers to a backyard structure without a full kitchen, used only for visitors. A true ADU has a complete kitchen, making it a fully independent dwelling. If you're building new, always include a full kitchen — it preserves all your future options.
What is a mother-in-law suite in Arizona?
An ADU used specifically to house a family member — typically an aging parent. It can be a detached backyard casita or an addition to the main home. The term describes how the unit is used, not a different type of structure.
Can I build a granny flat in Phoenix?
Yes. Under HB 2720 (effective January 2025), Phoenix must allow at least one detached ADU on any single-family lot. Most Phoenix lots qualify, subject to setbacks, lot size, and HOA rules. Check your eligibility here.
Last updated: September 2026. Arizona ADU law continues to evolve. For the most current rules on what you can build and where, see our Arizona ADU Guide.
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