What is an ADU in Arizona and what are the rules?

An ADU, short for accessory dwelling unit, is a small, self-contained second home built on the same lot as a single-family house. It has its own place to sleep and its own bathroom, and it usually has its own kitchen. State law in A.R.S. 9-461.18 defines it as a living unit on the same lot as a larger single-family home, with its own sleeping and sanitation facilities. As of 2025, Arizona cities with more than 75,000 people must allow at least one attached and one detached ADU on any lot where a single-family home is allowed. People in Arizona often call an ADU a casita, a guest house, or a granny flat.

The rules changed fast. A new state law forced big cities like Phoenix and Scottsdale to open their backyards to ADUs, and counties followed in 2026. Below is what an ADU is, what the law now requires, and how it differs from a plain casita or guest house.

What counts as an ADU in Arizona

An ADU is a complete, separate living space on a single-family lot, smaller than the main house. The state definition in A.R.S. 9-461.18 is plain: it is "a self-contained living unit" on the same lot as a single-family dwelling of greater square footage, "that includes its own sleeping and sanitation facilities and that may include its own kitchen facilities." So the minimum is a bedroom area and a bathroom. A kitchen is allowed but not strictly required by the state definition.

ADUs come in two basic forms:

• Attached ADU. Built onto the main house or carved out of it, like a converted garage with its own entrance, or an added suite. It shares a wall with the primary home.
• Detached ADU. A free-standing building in the yard, like a backyard casita or a unit above a detached garage. This is the most common image people have of a casita.

The key feature that makes it an ADU and not just an extra room is independence. An ADU has its own entrance and can function as a separate household. In most Arizona cities it can be rented out on its own, which is the legal line that separates a true ADU from a guest house.

What the Arizona ADU law requires

Arizona now forces larger cities and all counties to allow ADUs, and it caps the restrictions they can put on you. Two statutes control this. A.R.S. 9-461.18 covers cities with more than 75,000 people. A.R.S. 11-810.01 covers counties. Both grew out of House Bill 2720, signed in 2024, which a follow-up law (House Bill 2928, 2025) expanded.

Here is what the state requires the city or county to allow:

• At least one attached and one detached ADU on any single-family lot.
• One more detached ADU on lots of one acre or larger, if at least one unit on the lot is a restricted-affordable (deed-restricted) home.
• An ADU size of at least 75% of the main home's gross floor area or 1,000 square feet, whichever is less. A city cannot cap you below that floor.

The law also blocks several common roadblocks. A city or county cannot require extra parking for an ADU. It cannot set rear or side setbacks greater than five feet from the property line for the ADU. It cannot require a family or other relationship between the people in the main house and the people in the ADU. And it cannot ban renting the ADU as separate long-term housing.

Cities had to adopt these rules by January 1, 2025, and counties by January 1, 2026. If a city or county missed the deadline, the statute says ADUs "shall be allowed on all lots or parcels zoned for residential use" with no local limits at all. That penalty pushed nearly every covered government to act on time rather than hand homeowners an unlimited right to build.

The law does leave cities room on a few items. They can still require a building permit, enforce the building and fire code, require the ADU to connect to water and sewer or septic, and set reasonable height and lot-coverage limits. What they lost is the power to ban ADUs outright, demand a parking pad, force owner-occupancy as a blanket rule, or push the unit far from the lot line. The result is that a typical Phoenix or Scottsdale homeowner with a single-family lot now has a clear legal path to add one rentable backyard unit.

How Phoenix, Scottsdale, and Maricopa County apply it

Each city writes the state floor into its own zoning code, so the fine print varies. The big metro players have all updated their rules. Phoenix allows two ADUs on a standard single-family lot, plus a third on lots of at least one acre when one unit is affordable, under Phoenix Zoning Ordinance 706. Detached units in a required yard are capped near 15 feet in height.

Scottsdale adopted Ordinance 4687 on September 30, 2025. It allows one attached and one detached ADU on a normal lot, and a third (restricted-affordable) detached unit on one-acre lots. Scottsdale caps the first ADU at 1,000 square feet or 75% of the main home, whichever is less, and any second or third ADU at 500 square feet. A detached unit can sit as close as five feet to a side or rear lot line, and must stay at least ten feet from the main house.

Maricopa County updated its zoning ordinance to take effect January 9, 2026, which allows multiple ADUs on single-family lots in unincorporated areas like the Rio Verde Foothills. This matters for rural acreage parcels that sit outside any city. Always check your specific city or county code, because setbacks, height, and design rules differ block to block.

ADU versus casita versus guest house

A casita is just the everyday Arizona word for an ADU, but a "guest house" can be a legally different thing. People use all three terms loosely. The legal difference is whether the unit can be rented separately from the main home.

• A casita usually means a detached ADU. It is a real, rentable second home with its own kitchen and bath. Most new backyard casitas built under the 2025 law are ADUs.
• A guest house in some city codes is a detached room for the homeowner's guests that cannot be rented on its own. Scottsdale, for example, treats a Guest House as a separate category that is "not a dwelling unit or an Accessory Dwelling Unit" and may not be rented apart from the main residence.

So the practical test is simple. If you want a unit you can lease to a tenant for income, you want an ADU (casita) built to the ADU rules. If you only want extra space for visiting family and do not plan to rent it, a guest house may be cheaper and face fewer requirements. Confirm the exact definitions with your city, since Phoenix, Scottsdale, and Maricopa County each draw the line a little differently.

Where Jematell Homes comes in

Setbacks, overlays, and ADU rules shape our design work from day one. Reach out and we will talk through how this applies to your specific lot and plan.

Sources

• A.R.S. 9-461.18 (Accessory dwelling units; regulation; municipalities)
• A.R.S. 11-810.01 (Accessory dwelling units; counties)
• City of Scottsdale: Accessory Dwelling Units (ADUs)
• Phoenix Zoning Ordinance 706 (Accessory Uses and Structures)