Casita vs guest house vs ADU: what's the difference in Arizona?

The three words overlap, but in Arizona they are not the same thing. An ADU (accessory dwelling unit) is the legal term for a self-contained second home on your lot that you are allowed to rent. A casita is the everyday Southwest word people use for that same small building. A guest house is a separate, stricter category that some cities, including Scottsdale, define on their own: it cannot be rented separately, it is capped at a smaller size, and it often skips the full kitchen. So the gap that matters most is legal, not architectural. ADU and casita usually describe a rentable unit. A guest house, by city rule, is not rentable.

Getting these straight matters because it changes what you can build, how big it can be, and whether it can ever earn rent. Below is each term, then a clean side-by-side.

ADU: the legal, rentable unit defined by state law

An ADU is the term Arizona state law uses, and it is the one that lets you rent. Under A.R.S. 9-461.18, an accessory dwelling unit is "a self-contained living unit that is on the same lot or parcel as a single-family dwelling of greater square footage than the accessory dwelling unit, that includes its own sleeping and sanitation facilities and that may include its own kitchen facilities." The key words are "self-contained" and "may include its own kitchen." An ADU is built to function as a real, independent home.

State law also protects your right to build and rent one. A.R.S. 9-461.18 bars cities over 75,000 people from setting rear or side setbacks "more than five feet from the property line," from requiring "additional parking," and from prohibiting the unit's use "as separately leased long-term rental housing." For counties, A.R.S. 11-810.01 sets a parallel rule. So an ADU is the category that carries the legal right to rent the unit out, which is the whole reason most owners build one to begin with.

Size is capped by the same statute: an ADU may be up to "seventy-five percent of the gross floor area of the single-family dwelling on the same lot or parcel or one thousand square feet, whichever is less." That ceiling, plus the rental right, is what defines an ADU in the eyes of the law.

Casita: the everyday word for the same building

A casita is not a separate legal category. It is the Southwest's everyday word, Spanish for "little house," for a small second living space on your lot. In practice, when an Arizonan says casita, they usually mean what the code calls an ADU: a detached or attached unit with its own entrance, a bathroom, and often a kitchen. The word is about feel and local custom, not a line in the statute.

That is why your city's planning department will not have a permit form that says "casita." It will have forms for an ADU or a guest house, and your casita gets built under one of those legal labels depending on what it is. A casita with a full kitchen that you can rent is an ADU. A casita with no kitchen that you cannot rent is, in a city like Scottsdale, a guest house.

So treat casita as the friendly name and ADU or guest house as the legal name. When you go to permit and build, the rules that apply come from the legal category, not from the word casita. For the everyday meaning and uses, see our glossary entry on the casita.

Guest house: the stricter, non-rentable category

A guest house is a separate, more limited category that some Arizona cities define on their own, and the defining rule is that you cannot rent it. The City of Scottsdale states it plainly: "A Guest House may not be rented separately from the main single-family residence on a property." That single rule is the heart of the difference. An ADU can be leased. A Scottsdale guest house cannot.

Scottsdale also caps a guest house smaller and ties its utilities to the main home. The guest house must be "no greater than one-half (1/2) the gross floor area of the main single-family residence on the same property," which is tighter than the ADU's 75 percent or 1,000 square feet. Scottsdale's own comparison lists guest houses as detached-only, non-rentable, and required to share utilities with the main residence, while ADUs may be metered separately. A guest house also typically skips the full kitchen, which is part of what keeps it from functioning as an independent rental.

The practical payoff is cost. Because a guest house has no second kitchen and shares the main home's utilities, it skips the extra plumbing, the gas line, and a separate meter that a full ADU needs. That makes it cheaper to build. The trade-off is that it earns no rent. If you only need room for visiting family or a home office, a guest house is the lower-cost path. If you want rental income, you need a full ADU.

Side by side: which one fits your goal

Start with one question to choose the right category: do you want to rent it out. Your answer points to the category, and the category sets the size and the cost. Here is the practical breakdown for Arizona, using Scottsdale's rules as the city example since they spell out all three:

• Want rental income, full independent unit. Build an ADU. State law (A.R.S. 9-461.18) protects your right to rent it, allows a full kitchen, caps size at 75% of the home or 1,000 square feet, and limits setbacks to five feet. Most people call this a casita.
• Just need space for family or an office, want lower cost. Build a guest house where your city offers the category. In Scottsdale it cannot be rented, caps at 50% of the main home, shares utilities, and often has no full kitchen, which saves real money.
• Using the word casita. That is the local name for either of the above. Confirm with your city whether you are permitting it as an ADU or a guest house, because the rules and rental rights come from that legal label.

One more local note: these rules vary by jurisdiction. Scottsdale spells out a distinct guest-house category, but not every Arizona city or county does, and a county lot follows A.R.S. 11-810.01 instead of the city statute. Always confirm the exact size limit and rental rule with your city or county before you design, since that one answer drives the budget.

We build casitas, ADUs, and guest houses across Scottsdale, Phoenix, Cave Creek, and the surrounding metro, and we are clear from the start about which legal category your project falls under and what that means for kitchen, size, and rental rights. For full state and city ADU standards, see Arizona's ADU requirements, and for budgeting, see the cost to build a casita in Arizona. If a detached guest house or casita is what you have in mind, that is exactly the kind of build-on-your-lot project we handle.

Building with Jematell Homes

Setbacks, overlays, and ADU rules shape our design work from day one. We would rather answer your questions before you build than after, so get in touch any time.

Sources

• A.R.S. 9-461.18 (Accessory dwelling units; municipalities; definition)
• A.R.S. 11-810.01 (Accessory dwelling units; counties)
• City of Scottsdale: Guest Houses (definition and limits)
• City of Scottsdale: Accessory Dwelling Units (size standards)