A.R.S. 9-461.18: Arizona's City ADU Mandate

For years, building a backyard casita in a Phoenix-area city meant fighting local zoning that either banned a second home outright or buried it in conditions. A.R.S. 9-461.18 flipped that. The statute, in effect since 2025, tells every Arizona city with more than 75,000 residents that it must allow accessory dwelling units (a second, smaller home on a single-family lot, often called a casita or guest house) by right. Cities can still set reasonable rules, but the law strips out the conditions that used to kill these projects.

Which cities this binds and what they must allow

The mandate applies only to cities over 75,000 people. In the metro area that covers Phoenix, Scottsdale, Mesa, Chandler, Gilbert, Glendale, Peoria, and Surprise. Smaller towns like Cave Creek, Carefree, and Fountain Hills are not forced to comply, though some opt in. A covered city has to permit at least one attached and one detached ADU on any lot zoned for a single-family home. It must treat that ADU as a standard use, not a special exception you have to argue for at a hearing.

A municipality shall allow at least one attached and one detached accessory dwelling unit on any lot or parcel that contains a single-family dwelling.

The law also protects a usable size. A city must allow an ADU of at least 75 percent of the main home's floor area or 1,000 square feet, whichever is less. On a lot of one acre or more, the city must allow a second detached ADU if one of the units is rented at a restricted-affordable rate.

What this means for you

If you own a single-family lot in a covered city, you have a legal right to add a casita. The city cannot stack on the hurdles that used to stop you. It cannot require you to live in either unit. It cannot require the renter to be a family member. It cannot demand extra parking. It cannot make the casita's exterior match the main house. And it cannot apply setbacks stricter than your single-family zone, where rear and side setbacks are capped at five feet from the property line. The city also cannot make you sign restrictive covenants or install commercial fire sprinklers just because the structure is an ADU.

Here is a concrete case. You own a 9,000-square-foot lot in Scottsdale with a 2,800-square-foot home. The city has to let you build a detached casita of up to 1,000 square feet, since 1,000 is less than 75 percent of 2,800. It cannot require added parking or demand that a relative live there. You can rent it long-term, meaning a lease of 90 days or longer. The one thing the city keeps control of is short-term rentals, which fall under separate rules.

If a covered city failed to adopt the required regulations by January 1, 2025, the penalty falls on the city, not you: ADUs become allowed on every residentially zoned lot without the city's local limits. The statute does not reach tribal land or property inside certain military and commercial airport noise zones above 65 decibels.

How this connects to the rest of the law

This section is the city side of a two-statute pair. The county counterpart is A.R.S. 11-810.01 , which applies the same idea to unincorporated land with a January 1, 2026 deadline, so which statute governs your build depends on whether your lot sits inside city limits. A smaller attached unit carved inside the main house is a junior accessory dwelling unit , and the detached backyard version is what most Arizonans call a casita . For the applied questions, see what is an ADU in Arizona , the build details in Arizona ADU requirements , and the count limits in how many ADUs per property in Arizona .

Full text and source

Read the current version, including the full list of prohibited conditions and the affordable-rate provision, on the legislature's site: View A.R.S. 9-461.18 on azleg.gov .

Because cities can still impose height, setback, and design rules within the limits this section sets, confirm your specific lot's ADU allowance with your city planning counter before you commission casita plans.