ADU, guest house, or short-term rental: which one are you building?

1. The question, as people ask it

"Is a casita the same as an ADU?" — also asked as "Can I rent out my guest house?", "Do I need an ADU permit for a casita I'll put on Airbnb?", and "What's the difference between a guest house and an accessory dwelling unit in Arizona?"

2. The short answer

They are three different legal things, and the label on your permit decides what you may do with the building.

• An accessory dwelling unit (ADU) is a self-contained second home on the same lot as your house. State law defines it as "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" (A.R.S. § 9-461.18(I)(1); same words in § 11-810.01(J)(1)). Cities over 75,000 and all counties must allow at least one attached and one detached ADU per single-family lot, and may not stop you from renting it separately as long-term housing (§ 9-461.18(A), (B)(1); § 11-810.01(A), (C)(1)).
• A guest house is a local zoning category, not a state one. Where it exists, it is a room or structure for family or guests that is not a separate rental. Maricopa County defines it as "A habitable space within a separate structure on the same lot as the primary dwelling unit used to house family or guests without compensation" (MCZO Chapter 2). Scottsdale's ADU page draws the same line: guest houses may not be rented separately, share the main house's utilities, and are detached only, while ADUs may be rented separately, need their own metered utilities, and may be attached or detached.
• A short-term rental (STR) is a use, not a building type. State law defines a vacation rental or short-term rental as a house or one-to-four-family dwelling "that is also a transient public lodging establishment or owner-occupied residential home offered for transient use" (§ 9-500.39(L)(4)(a)). Cities and counties "may not prohibit vacation rentals or short-term rentals" (§ 9-500.39(A); § 11-269.17(A)), but they may license them and, in one specific case, may require you to live on the property.

So: a guest house cannot be a separate rental at all. An ADU can be a long-term rental by right. An ADU used as a short-term rental is allowed by state law but may trigger an owner-residency requirement and a local license.

3. Why you may have heard otherwise

"Casita" is not a legal term. Builders, listings, and city web pages use it for guest houses and ADUs alike. Your permit will say one or the other. If your plans are approved as a guest house, you did not get ADU rights, and renting it separately can be a zoning violation.

"The state legalized casita Airbnbs." What the state did is narrower. Arizona's STR preemption predates the ADU law. The 2024 ADU law added a clause letting cities require owner residency for an STR on a property with an ADU; the 2025 follow-up rewrote the trigger. Today a city may require the owner "to reside on the property if the property contains an accessory dwelling unit and if a certificate of occupancy, certificate of completion or similar final approval for the accessory dwelling unit was issued by the municipality on or after September 14, 2024" (§ 9-500.39(B)(9)). An ADU with final approval on or before September 13, 2024 is outside that rule. Counties have the identical power (§ 11-269.17(B)(9)).

"Long-term and short-term are treated the same." They are not. The ADU statute defines long-term rental as "rental use in which the tenant holds a lease of ninety days or longer or on a month-by-month basis" (§ 9-461.18(I)(3)) and forbids cities from prohibiting it. Short-term use runs under the separate STR statute with its license, notice, insurance, and residency provisions.

4. How this plays out in three jurisdictions

Unincorporated Maricopa County. ADUs are a permitted use on any single-family lot, at least one attached and one detached, "leased or occupied independently of the principal dwelling as long-term rental housing" (MCZO Article 602.3.1–.4). A guest house is a separate defined term for family or guests "without compensation." For short-term rentals: an STR is a rental of "no more than 30 consecutive days"; "No more than one ADU may be used as a short-term rental"; "Only one (1) STR unit may be rented per parcel at any given time"; and "The owner of a short-term rental must reside on the property if the property contains an accessory dwelling unit unless a certificate of occupancy, certificate of completion or similar final approval for the accessory dwelling unit was issued by the County on or before September 13, 2024" (Articles 602.2.1–.4, 602.3.7).

Scottsdale. Ordinance 4687 (September 30, 2025; Zoning Ordinance Sec. 7.900) allows attached, detached, and restricted-affordable detached ADUs, requires separately metered electric and gas, a separate entrance and address, and caps the first ADU at 1,000 square feet or 75% of the main house. The city's page states: "The property owner must reside on the premise for any ADU that is rented, leased, offered for rent or lease, or occupied as a Vacation Rental or Short-term Rental." The Sec. 7.900 ordinance text was not read for this page, and that statement is broader than the state STR trigger in § 9-500.39(B)(9); we report it as the city states it. STR licensing is under Sec. 7.203.

Phoenix. Ordinance G-7317 (Z-TA-2-24-Y, adopted November 20, 2024) allows one attached and one detached ADU per lot, with a third on one-acre lots that include an affordable unit, and caps an ADU at 75% of the main house's floor area and 1,000 square feet on lots up to 10,000 square feet. On lots larger than 10,000 square feet, the cap is 3,000 square feet or 10% of net lot area, whichever is less. The staff report proposed removing the city's earlier prohibition on using an ADU as a short-term rental and handling owner residency through the city's STR licensing code. We did not read Phoenix's guest-house definition or its current STR ordinance for this page.

5. What's still unsettled

• "Similar final approval." The September 14, 2024 trigger turns on a certificate of occupancy, certificate of completion, "or similar final approval." What counts for an older casita with no certificate on file is not answered in the statute.
• Converting a guest house to an ADU. Whether an existing guest house can be re-permitted as an ADU, and what utility separation or fire-separation work that requires, is a local building-code question we did not research.
• Phoenix guest-house rules and STR code. Not read; see above.
• HOA covenants. Private CC&Rs can restrict any of the three. State law leaves them in place for ADUs (§ 9-461.18(C)).

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• A.R.S. § 9-461.18 (ADU definition, long-term rental, covenants): https://www.azleg.gov/ars/9/00461-18.htm
• A.R.S. § 11-810.01 (county ADU statute): https://www.azleg.gov/ars/11/00810-01.htm
• A.R.S. § 9-500.39 (municipal STR statute; (B)(9) owner residency): https://www.azleg.gov/ars/9/00500-39.htm
• A.R.S. § 11-269.17 (county STR statute; (B)(9)): https://www.azleg.gov/ars/11/00269-17.htm
• Maricopa County Zoning Ordinance (Chapter 2 definitions; Articles 602.2–602.3): https://esd.maricopa.gov/DocumentCenter/View/4785
• City of Scottsdale, Accessory Dwelling Units: https://www.scottsdaleaz.gov/codes-and-ordinances/accessory-dwelling-units
• City of Phoenix staff report Z-TA-2-24-Y (September 30, 2024): https://phoenix.legistar.com/View.ashx?GUID=E4CD7514-68AE-4ECB-96CA-990ED7B77730&ID=13552322&M=F

This page reports what the statutes and ordinances say. It is not legal advice. Confirm the permit category and current requirements with your city or county before you design or build.

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