The Arizona ADU & Casita Guide

What changed in 2025

For years, whether you could put a casita in your back yard came down to your city's zoning code, and a lot of Valley cities either banned them outright or buried them under conditional-use processes that made them impractical.

Arizona House Bill 2720 changed that. Signed in May 2024 and effective January 1, 2025, it requires any Arizona municipality with a population above 75,000 to permit accessory dwelling units on lots zoned for single-family residential use. Specifically, covered cities must allow:

• At least one attached and one detached ADU on a single-family lot;
• On lots of one acre or more, at least one additional detached ADU, provided one of the units is designated as a restricted-affordable dwelling.

The statute also restricts several tactics cities had used to discourage ADUs. Covered municipalities generally may not require that the ADU match the exterior design of the main house, may not prohibit the ADU from being leased separately from the primary residence, and are limited in the setback and owner-occupancy requirements they can impose.

A follow-up bill in 2025 extended broadly similar requirements to unincorporated county land, which matters for parts of the West Valley. Communities like Sun City, Sun City West and Anthem are unincorporated Maricopa County rather than incorporated cities.

Two caveats worth saying plainly. First, every covered city wrote its own compliant ordinance, so the details (size caps, height limits, setbacks, parking) differ from Surprise to Scottsdale. Second, the law governs zoning. It does not touch building code, and it does not touch private HOA covenants. More on that below.

ADU, casita, guest house: what is the difference?

These words get used interchangeably in marketing and precisely at the permit counter, which is where the confusion starts.

The kitchen is the hinge. Add a kitchen and you have created a dwelling unit, which changes the permit path, the code requirements, the utility sizing and often the tax assessment. Leave it out and you have built a very nice detached room. Neither is wrong, but decide deliberately, because retrofitting a kitchen later costs far more than including it from the start.

Will my lot actually work?

The law says your city has to allow an ADU. It does not say your particular lot can fit one. Five things decide that, and we check all of them on the first site visit:

• Setbacks. How close the structure can come to each property line. This is the constraint that kills more back-yard casitas than anything else, particularly on the narrow-and-deep lots common in newer West Valley subdivisions.
• Lot coverage. Most zoning codes cap the percentage of the lot that can be covered by structures. If your house and existing garage already use most of it, the casita has to shrink.
• Easements. Utility, drainage and access easements run through a surprising number of Valley back yards, and you generally cannot build over them. These are on your plat, not always on your survey.
• Access. Something has to get a concrete truck, a lumber delivery and eventually a framing crew into the back yard. A six-foot side gate between two houses is a real cost problem.
• Drainage. Adding a roof and a slab changes where water goes in a monsoon. Some jurisdictions want a drainage plan, and you want one regardless.

The HOA question, which is the one people get wrong

This is the most common misunderstanding about the new law, and it is an expensive one.

HB 2720 constrains what a municipality can require through its zoning code. An HOA is not a municipality. Its CC&Rs are a private contract you agreed to when you bought the house, and the state ADU statute does not rewrite that contract. Your HOA can still have architectural standards, height limits, exterior material requirements, and in some cases restrictions that make an ADU impractical.

Where an HOA does allow it, budget real time for the architectural review committee. In the master-planned communities we work in most (Verrado, Vistancia, Sun City, Sun City West, Anthem) HOA review frequently takes longer than city plan review, and the two run in sequence rather than in parallel, because most cities want to see the HOA approval in the submittal package.

What an ADU actually costs in Arizona

Our published 2026 range is $150,000 to $1,000,000+. That is wide because "ADU" covers wildly different projects. Here is roughly how they break down:

Notice that square footage is not the organizing variable. A 700 sq ft detached casita costs more than a 900 sq ft garage conversion, because the conversion starts with a foundation and a roof already paid for. Full detail and what moves each number lives on the casita and ADU cost page.

How long it takes

• Design and engineering: four to eight weeks. Floor plan, elevations, structural engineering, and enough detail to submit.
• HOA architectural review: two to eight weeks, and it varies enormously by community. Start this early.
• City plan review and permit: four to twelve weeks depending on jurisdiction and whether corrections come back.
• Construction: three to five months for a detached casita, less for a conversion.

Call it four to eight months end to end. Anyone promising a detached, permitted casita in ninety days from first phone call is either not permitting it or not telling you the truth.

Utilities, panels and the things that surprise people

Most ADUs extend the existing utility services rather than adding new meters, which is much cheaper. The question is whether what you have can carry the load.

The one that catches people is the electrical panel. A lot of West Valley homes built in the 1990s and early 2000s have 150- or 200-amp service that is already close to fully loaded once you count the main house's HVAC, pool equipment and EV charger. Adding a second dwelling with its own air handler and range can push you over, and a service upgrade is a five-figure line item that never appears in a casual estimate. We do a load calculation before we quote, not after.

Water and sewer are usually more forgiving, but the trench run from the main house to the back of the lot is real work, and if you are on septic rather than sewer, adding a bedroom count can require a system evaluation or upgrade.

Five expensive mistakes we watch people make

1. Assuming the state law overrides the HOA. It does not. Check the CC&Rs first.
2. Skipping the kitchen to save money, then wanting one later. Rough-in the plumbing and gas even if you defer the finish. Retrofitting into a finished slab is brutal.
3. Buying a prefab unit before checking setbacks. The unit is the easy part. Foundation, utilities, permits, crane access and site work often cost more than the box.
4. Budgeting on rental income that the HOA prohibits. Confirm rental rules in writing before the pro forma becomes the plan.
5. Not checking the panel. See above. It is the single most common five-figure surprise on ADU projects.