What are the ADU setback and size rules by city in Arizona?

Across Arizona's larger cities the ADU setback and size rules rhyme but do not match. Phoenix, Scottsdale, and Mesa all permit a detached accessory dwelling unit (ADU), or casita, as close as five feet to a side or rear lot line, cap the first unit near 1,000 square feet, and cannot require extra parking. Where they diverge is rentals and height. Scottsdale makes you live on site to rent, Phoenix bans short-term rentals, and Mesa sets no owner rule at all. Smaller towns under 75,000 people, like Cave Creek, Fountain Hills, and Carefree, are exempt from the state mandate and treat casitas as guest space you cannot rent separately. This page lays the rules side by side so you can compare your city in one place.

The reason the big cities look similar is a shared floor. State law A.R.S. 9-461.18 sets minimum ADU rights for any city over 75,000 people. Each city then adds its own detail on top. The reason small towns look different is that the same statute exempts them. Below is the comparison, rule by rule.

ADU setback and size: the city-by-city comparison

The covered cities share a five-foot setback and a roughly 1,000-square-foot first-unit cap, but the size details and rental rules differ. Here is the side-by-side for the three large cities Jematell serves most, plus the exempt small towns:

City	ADUs per lot	First-unit size cap	Rear/side setback	Extra parking	Rent / owner rule
Phoenix	2 (3 on 1+ acre w/ affordable)	1,000 sq ft on lots up to 10,000 sq ft; larger on bigger lots	5 ft	None required	Long-term rental OK; no short-term rental
Scottsdale	1 attached + 1 detached (3rd on 1+ acre)	1,000 sq ft or 75% of home; 2nd unit 500 sq ft	5 ft	None required	Must live on site to rent
Mesa	1 attached + 1 detached (3rd on 1+ acre)	1,000 sq ft or 75% of home's interior area	5 ft	None required	No owner-occupancy rule
Cave Creek	Detached living quarters in Desert Rural zones only	Set by zone and lot coverage	Per zone (large lots)	Per local code	Guest use; not a separate rental
Fountain Hills	Guest house allowed	Accessory use	Per local code	One space per guest house	Cannot be rented
Carefree	Guest house allowed	Accessory structure, 16 ft tall max	Per local code	Per local code	Cannot be leased separately

The table shows the pattern at a glance. The three big cities give you a rentable unit with a five-foot setback and no parking penalty. The three small towns give you guest space you cannot lease on its own. Use it as a starting map, then read your city's full rule for the fine print, since base zoning districts change the height and yard numbers.

Where the size caps actually differ

The 1,000-square-foot cap is the common headline, but how each city gets there is not the same. The state floor caps an ADU at 75% of the main home or 1,000 square feet, whichever is less, and every covered city honors that minimum. The differences live in the details:

• Phoenix ties the cap to lot size. On a lot up to 10,000 square feet, the ADU caps at 1,000 square feet. On a larger lot the cap rises to the lesser of 3,000 square feet or 10% of the net lot area, so a half-acre parcel can support a bigger unit.
• Scottsdale caps the first ADU at 1,000 square feet or 75% of the home, then caps any second or third unit at just 500 square feet. So a two-unit Scottsdale project pairs one larger casita with one compact unit.
• Mesa measures interior habitable area, not gross footprint, when applying the 75% or 1,000-square-foot test. An attached garage or covered patio does not count against the cap the same way.

So if maximizing square footage matters, the city you build in changes the answer. A large lot favors Phoenix's lot-size formula. A single bigger unit fits Scottsdale's first-unit cap. A conversion with attached non-living space can favor Mesa's interior-area measure. For full detail, see the Phoenix ADU rules, Scottsdale ADU rules, and Mesa ADU rules.

The setback and parking rules that are nearly the same

On setbacks and parking, the three covered cities are close to identical, because state law pins them. A.R.S. 9-461.18 bars a covered city from setting a rear or side ADU setback "more than five feet from the property line," so Phoenix, Scottsdale, and Mesa all allow a detached unit five feet off the rear and side lines. The same statute bars any rule that would "require that a lot or parcel have additional parking to accommodate an accessory dwelling unit," so none of the three can force a new parking pad.

The small differences are in spacing and conversions. Scottsdale adds a ten-foot separation between the ADU and the main home, plus a required private outdoor space of at least 50 square feet. Mesa requires the same five-foot setback for new construction but waives added setbacks when you convert an existing structure, and it does not make you replace covered parking if you turn a garage into a casita. Phoenix layers in a 15-foot height cap for a detached unit placed in a required yard, which is the strictest height rule of the three. So even where the floor is shared, the city-specific add-ons can decide your design.

How owner-occupancy and rentals split by city

The biggest real difference between Arizona cities is the rental and owner-occupancy rule, and it is where you should focus if income is the goal. The covered cities all allow some renting, but on different terms:

• Scottsdale requires the property owner to live on site for any ADU that is rented, leased, or used as a short-term rental.
• Phoenix allows long-term renting with no owner-occupancy requirement, but bans ADU short-term rentals outright.
• Mesa sets no owner-occupancy rule, so an investor or an owner-occupant can rent the unit on the same terms.

The exempt small towns are stricter than all three. Cave Creek allows detached accessory living quarters only in its large Desert Rural zones, and only as part of your own household. Fountain Hills and Carefree allow guest houses but bar renting them separately, since the state never forced the issue there. The lesson is that the rental rule, not the setback, is what most often makes or breaks an ADU plan, so check it first when you compare two cities. For the full small-town picture, see whether Cave Creek, Fountain Hills, and Carefree allow casitas and ADUs.

Rules like these shift over time, and your base zoning district controls the exact height and yard numbers, so confirm the current standard with your city or town before you design. For the statewide framework that ties it all together, see Arizona's ADU requirements and which Arizona cities are required to allow ADUs.

How Jematell Homes helps

Setbacks, overlays, and ADU rules shape our design work from day one. We would rather answer your questions before you build than after, so get in touch any time.

Sources

• A.R.S. 9-461.18 (Accessory dwelling units; municipalities; population threshold)
• Phoenix Zoning Ordinance Section 706 (Accessory Uses and Structures)
• City of Scottsdale: Accessory Dwelling Units (ADUs)
• City of Mesa Ordinance 5883 (Accessory Dwelling Units; Section 11-31-3)