Arizona ADU Laws: Casita Rules for Phoenix and Big Cities

Arizona's ADU laws come down to one statute: HB 2720, signed in May 2024 and now A.R.S. § 9-461.18. It requires every city or town of more than 75,000 people, such as Phoenix, Tucson and Mesa, to allow at least one attached and one detached accessory dwelling unit (a "casita") on each lot zoned for a single-family home. Cities cannot require the occupants to be related, demand extra parking or require the casita to match the house, and must allow a unit of at least 1,000 ft² or 75% of the house, whichever is less. Cities that missed the January 1, 2025 deadline for compliant rules must allow ADUs without limits, and since 2025 a separate statute does the same for county land. Law current as of October 2026.

This post covers what the statute requires, which cities it covers, what local rules still apply, and an 800 ft² detached casita you can open and adapt. For the other states that require cities to allow ADUs, see ADU laws by state.

Who the law covers

The statute applies to municipalities with a population of more than 75,000. In practice that is the large Phoenix-area cities (Phoenix, Mesa, Chandler, Gilbert, Glendale, Scottsdale, Peoria, Tempe, Surprise, Goodyear and others), Tucson, and a handful of others such as Yuma and Flagstaff, depending on current population figures. Smaller towns are not covered by § 9-461.18, though many allow casitas under their own codes. Unincorporated county land now has its own rule: HB 2928 (2025) added A.R.S. § 11-810.01, which requires every county to allow one attached and one detached ADU on single-family lots, with an extra detached unit on lots of an acre or more and exceptions near airports and military installations. Counties that had not adopted rules by January 1, 2026 must allow ADUs without limits.

What cities must allow, and may not require

The size rule is a floor on what a city must allow, not a cap on what you can build: a city may allow larger casitas. For a 1,600 ft² house, 75% is 1,200 ft², so the city must allow at least 1,000 ft²; for a 1,200 ft² house, it must allow at least 900 ft².

Phoenix and Tucson before and after

Both large cities already allowed casitas before HB 2720. Phoenix adopted an ADU ordinance in September 2023 allowing one ADU per single-family lot, up to 1,000 ft² on lots up to 10,000 ft², and banned their use as short-term rentals, a ban that state law has since overtaken for many casitas. Tucson amended its code in October 2021 to allow one ADU on any residential lot, up to 750 ft² on lots under 7,000 ft² and 1,000 ft² on larger lots (Tucson PDSD). Both had to bring those rules into line with HB 2720, which requires two units and limits setbacks, so check their current codes rather than the original ordinances.

An 800 ft² detached casita

The detached unit the statute guarantees is the classic backyard casita, and FloorGen's 800 sq ft 2-bed ADU template is one: a detached unit 40 × 20 ft to the outside of the walls (800.1 ft² gross) with 700.5 ft² of net floor area inside them. Two bedrooms sit at one end, a bath and laundry alcove in the middle, and an open kitchen, dining and living room at the other.

At 800 ft² the casita is under the 1,000 ft² floor, so a covered city must allow it on size behind any house of about 1,070 ft² or more (75% of 1,070 is just over 800). Behind a smaller house the guarantee is 75% of the house: 750 ft² for a 1,000 ft² house, so shorten the plan by a couple of feet or ask whether your city allows more. A 20 ft deep footprint fits behind most suburban houses with the 5 ft side and rear setbacks the law allows. In the desert, orient the big windows north or east, keep west walls mostly solid, and shade the door; the windows can be moved in the editor to suit your lot.

The other route is an attached casita in the house's two-car garage, which the ban on extra parking requirements makes easier. A 20 × 20 ft garage holds a studio of about 360 ft² net; the garage conversion ideas post lays one out.

Building code: what plan review checks

Arizona has no statewide residential building code; each city adopts an edition of the International Residential Code with local amendments. A casita is reviewed like a small house:

• Habitable rooms other than kitchens of at least 70 ft² and 7 ft in any direction (2021 IRC R304; 2024 IRC R312).
• Ceiling height of at least 7 ft in habitable rooms (2021 IRC R305); check this in a garage with a sloped slab.
• An emergency escape and rescue opening in each sleeping room (2021 IRC R310), including a studio's sleeping area.
• Fire separation between an attached casita and the rest of the house or any remaining garage, and smoke and CO alarms.
• The energy code: an uninsulated garage needs insulation and a conditioned envelope.

The statute bars cities from requiring fire sprinklers for ADUs, but other fire and building rules still apply. FloorGen's IRC profile checks room areas and widths, ceiling heights, bedroom escape windows, stairs and door swings in the plan; it does not check fire separation, setbacks or the energy code. The garage conversion guide covers conversion-specific issues such as the slab and the old door opening, and how to draw plans for a permit covers the drawing set. Arizona's rules are close to Washington's two-ADU law and to Florida's new by-right ADU law, if you want to compare. Nearby, Utah's SB 284 and Colorado's HB24-1152 take a narrower approach, with one ADU per lot.