How many ADUs can I have on my property in Arizona?

Two accessory dwelling units, one attached and one detached, is the floor Arizona law sets for a single-family lot, and most homeowners can plan on that number. On a lot of one acre or more, you can add a third detached ADU if at least one unit on the lot is a deed-restricted affordable home. This floor comes from A.R.S. 9-461.18 for cities over 75,000 people and A.R.S. 11-810.01 for counties. Your city can allow more, but it cannot allow fewer. So a typical Phoenix or Scottsdale homeowner can plan on two ADUs, and a rural acreage owner on up to three.

The exact count depends on whether your property is inside a covered city, in unincorporated county land, and how big the lot is. Below is how the math works for a real Arizona property.

The state minimum: two ADUs per lot

State law requires covered cities and counties to allow at least one attached and one detached ADU on any lot where a single-family home is allowed. The statute is direct. A government must permit "at least one attached and one detached accessory dwelling unit as a permitted use" on those lots. That is two ADUs, on top of your main house, as a baseline you can count on.

"Attached" and "detached" are the two forms:

• An attached ADU shares a wall with the main house. A converted garage with its own entrance or an added suite both count.
• A detached ADU is a free-standing building, like a backyard casita or a unit over a detached garage.

Because the law requires one of each, you can pair a garage conversion with a new backyard casita and have two legal rental units on a standard lot. The city cannot cap you at one, and it cannot demand a family relationship or extra parking for either unit. This is the floor that applies to most single-family lots in Phoenix, Scottsdale, and the other large Valley cities.

Here is a real example. Say you own a 7,500-square-foot lot in Phoenix with a three-bedroom house. Under the two-ADU floor, you could convert the attached garage into a one-bedroom attached ADU and build a small detached casita in the back yard. That gives you the main house plus two rentable units on one lot. A few years ago, most Valley cities would have blocked the second kitchen or the separate rental. Today the count is set by state law, and the city's job is to process your permit, not to decide whether you may have ADUs at all.

The third ADU on one-acre lots

On a lot of one acre or larger, the state lets you add a third ADU, but with a condition: at least one unit on the property must be restricted-affordable. The statute requires "a minimum of one additional detached accessory dwelling unit" on a lot of "one acre or more in size if at least one accessory dwelling unit on the lot or parcel is a restricted-affordable dwelling unit."

A restricted-affordable unit is one tied to a deed restriction. The restriction caps the rent or sale price so the unit stays affordable to lower-income households for a set number of years. In plain terms, you give up market rent on one unit to earn the right to build a third. So the third unit comes with strings. It fits owners who want maximum density on a large parcel and are willing to commit one unit to affordable housing.

This third-unit rule matters most on Arizona's larger lots. Think acreage in Cave Creek, Carefree, or the Rio Verde Foothills. On a quarter-acre Phoenix lot, the two-ADU floor is what applies, because the lot is well under an acre. Confirm the affordable deed-restriction terms with your city or county before you count on the third unit. The income limits and the length of the restriction are specific, and they bind for years.

One more point on counting. The three-ADU path requires the third unit to be detached. You cannot reach three by adding a second attached unit. The state floor is one attached plus one detached, then one more detached on a large lot. So a homeowner aiming for three units should plan two free-standing buildings in the yard plus one unit attached to or carved out of the main house.

How Phoenix, Scottsdale, and the county count

Each city writes the state floor into its own code, and the metro players all land at two ADUs for a standard lot. Phoenix, under Zoning Ordinance 706, allows two ADUs on a lot with one single-family home, and a third on lots of at least one acre when one unit qualifies as affordable. Phoenix also ties each ADU's size to the lot, so the count is two but the square footage scales with your land.

Scottsdale allows one attached and one detached ADU on a normal lot, and a third detached (restricted-affordable) unit on one-acre lots, under the ordinance it adopted in September 2025. It caps the first ADU at 1,000 square feet and any second or third at 500 square feet.

Unincorporated Maricopa County, which includes the Rio Verde Foothills, updated its zoning ordinance effective in January 2026 to allow multiple ADUs on single-family lots, matching the state mandate under A.R.S. 11-810.01. So a rural acreage owner outside any city has the same two-to-three ADU framework. The key is to confirm which jurisdiction controls your parcel, since a Rio Verde lot follows county rules, not Scottsdale's, even though it sits nearby. The simplest way to check is to look up your parcel on the county assessor site and confirm whether it shows a city or "unincorporated" jurisdiction. That one detail decides which code, deadlines, and setback rules apply to your ADUs.

What the count does not include

The two-ADU floor is a minimum, not a ceiling. A few things sit outside it. First, a city or county can allow more than the state requires. If your local code is more generous, follow the local number. Second, the count is per single-family lot. The law applies where a single-family home is allowed. The framework is built around houses, not existing apartment or multifamily parcels.

Third, a guest house is a different category in some cities. A guest house that cannot be rented on its own may not count against your ADU limit. Scottsdale treats a Guest House as a non-rentable structure, separate from an ADU, with its own size rules. So if you only want extra space for visiting family, a guest house can sit alongside your two ADUs without using up a slot.

So the honest answer for most Arizona homeowners is two ADUs on a standard single-family lot, and up to three on a one-acre lot with an affordable unit. Check your specific city or county code for the size, setback, and affordable-housing details, since those vary, but the count itself is now set by clear statewide law.

The Jematell Homes approach

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; municipalities)
• A.R.S. 11-810.01 (Accessory dwelling units; counties)
• Phoenix Zoning Ordinance 706 (Accessory Uses and Structures)
• Maricopa County Zoning Ordinance Update (ADU provisions)