What are the ADU and casita rules in Phoenix, Arizona?

Phoenix lets you build two accessory dwelling units (ADUs) on a normal single-family lot, plus a possible third on large lots. The rules live in Phoenix Zoning Ordinance Section 706. An ADU, often called a casita or guest house in Arizona, is a small second home with its own kitchen, bathroom, and entrance. On lots up to 10,000 square feet, each ADU is capped at 1,000 square feet. A detached ADU placed in a required yard can be up to 15 feet tall. The city cannot make you add parking. The big catch is that a Phoenix ADU cannot be used as a short-term rental, though you can lease it long term.

These standards came from a state law that forced large Arizona cities to open their backyards to second units. Phoenix wrote that law into Section 706 and added its own size and height details. Below is what each rule means for a real Phoenix build.

How many ADUs you can build in Phoenix

A standard Phoenix single-family lot can hold two ADUs, and some larger lots can hold three. The city's ADU guide states that when a lot has no more than one single-family detached primary home, two ADUs are permitted in addition to that home. This goes beyond the bare state floor, which only requires one attached and one detached unit. Phoenix chose to allow two units on a typical lot, which gives a homeowner real flexibility to add rental space or family housing.

The third unit is the exception, not the rule. A lot of at least one acre can add a third ADU when one of the units is a deed-restricted affordable home. That ties the bonus unit to keeping rent below a set income limit, so most standard suburban lots will plan around two units, not three.

This is a real shift from the old Phoenix code. For years the city treated a second kitchen as a red flag and limited backyard living space. Section 706 now gives a clear, by-right path to add up to two rentable units on land already zoned for a single-family home. You still pull permits and meet the building code, but the city can no longer say no to the unit itself.

Phoenix ADU size limits by lot size

Phoenix sizes ADUs by your net lot area, which is the unusual part of its code. The state floor caps an ADU at 75% of the main home or 1,000 square feet, whichever is less. Phoenix keeps the 75% test but then sets the square-foot cap by how big your lot is:

• Lots up to 10,000 square feet (net area): each ADU caps at 1,000 square feet.
• Lots over 10,000 square feet: the cap rises to the lesser of 3,000 square feet or 10% of the net lot area.

So a quarter-acre lot near the 10,000-square-foot line stays at the 1,000-square-foot cap, while a half-acre lot (about 21,780 square feet) could support a larger unit, up to 10% of the net area. This lot-size approach rewards bigger parcels with more buildable square footage, which fits the larger lots common in north Phoenix and the foothill areas.

The 75% rule still applies on top of the lot-size cap. State law sets the floor a city must allow at "an accessory dwelling unit that is seventy-five percent of the gross floor area of the single-family dwelling on the same lot or parcel or one thousand square feet, whichever is less." So if your house is 1,200 square feet, 75% is 900, and that lower number controls even on a small lot. Run both tests, the lot-size cap and the 75% cap, and use the smaller result. The size cap is also a budget cap, since a 700 to 1,000 square foot casita costs far less to build than a full custom home, even though its per-square-foot price often runs higher because you are building a full kitchen and bath into a small footprint.

Setbacks, height, and placement in Phoenix

A detached Phoenix ADU can sit as close as five feet to the rear and side lot lines, and caps at 15 feet tall when placed in a required yard. A setback is the minimum distance a building must sit from a property line. State law bars the city from requiring a rear or side setback greater than five feet, and Phoenix honors that floor. That lets you push a backyard casita close to the fence and keep usable yard space.

Height is where Phoenix is specific. Under Section 706, a detached ADU located within the required rear or side yard has a maximum height of 15 feet, unless you get use permit approval for more. If the unit is not in a required yard, it can match the height allowed for the main home. So the trade-off is simple: build close to the line and stay at 15 feet, or pull the unit toward the center of the lot and build taller.

A few placement rules round it out. The ADU cannot sit in the required front yard, and it must keep clear access for fire and utilities. Each unit needs its own entrance and address. Phoenix also cannot force the ADU to copy the main home's roof pitch or exterior finish, which protects you from an expensive forced match. Confirm your zoning district's exact yard and height numbers before you design, since base districts vary across the city.

Parking and rental rules for Phoenix ADUs

Phoenix cannot require extra parking for an ADU, but it does ban short-term rentals of the unit. The no-parking rule comes straight from state law, which says a city may not require additional parking to accommodate an ADU. That removes a barrier that used to kill many backyard projects on lots with no room for another parking pad.

The rental rule is the part Phoenix owners miss most. You can rent a Phoenix ADU as long-term housing to any tenant, with no requirement that you live on the property and no required relationship between you and the tenant. What you cannot do is run the ADU as a short-term rental on platforms like Airbnb. The city's ADU guide is direct that ADUs may not operate as short-term rentals, even though the main home on the same lot is not held to that limit. So the income model for a Phoenix casita is a long-term lease, not a nightly rental.

This makes Phoenix different from some nearby cities. Scottsdale, for example, allows an ADU as a short-term rental only if the owner lives on site, while Phoenix simply blocks ADU short-term rentals outright. The state law itself protects long-term renting, barring a city from any rule that would "prohibit the use or advertisement of either the single-family dwelling or any accessory dwelling unit located on the same lot or parcel as separately leased long-term rental housing." That is the income path Phoenix leaves open. If nightly-rental income is your goal, confirm the current rule with the Phoenix Planning and Development Department before you build, because short-term rental policy is the most likely piece to change. For the statewide baseline these city rules build on, see our pages on Arizona's ADU requirements for setbacks, size, and utilities and which Arizona cities are required to allow ADUs.

The Jematell Homes approach

Setbacks, overlays, and ADU rules shape our design work from day one. If you are planning a custom home in Scottsdale, Rio Verde, or the greater Phoenix metro, we are happy to walk through your project.

Sources

• Phoenix Zoning Ordinance Section 706 (Accessory Uses and Structures)
• City of Phoenix: Accessory Dwelling Unit (ADU) Guide
• A.R.S. 9-461.18 (Accessory dwelling units; municipalities)
• City of Phoenix: Accessory Dwelling Units (Planning and Development)