How do I add a casita or detached structure to an existing Arizona home?

Four things gate a detached structure on an existing Arizona lot: zoning that allows it, a design that fits the size and setback limits, extended water, sewer, and power, and a building permit pulled before you start. That is the path for adding a casita or other detached structure to an existing home. A casita that has a kitchen and a bathroom is an accessory dwelling unit (ADU), a second small home on the same lot. Under A.R.S. 9-461.18, Arizona cities with more than 75,000 people must allow at least one attached and one detached ADU as a permitted use on most lots zoned for single-family homes. A detached garage, shop, or pool house without a kitchen follows simpler accessory structure rules. The path depends on which one you are building.

Casita, guest house, or ADU: know what you are building

What you can build and how it is regulated depends on whether the structure is a full dwelling or just a room. The terms get mixed up, so start here:

• A casita or ADU is a complete small home. It has its own kitchen, bathroom, and sleeping area, and someone could live in it full time. This is what the state ADU law covers.
• A guest house typically has a bathroom and sleeping space but no full kitchen. Some cities limit it so it cannot be rented as a separate home.
• A detached accessory structure is a garage, workshop, studio, or pool house with no kitchen and no full bathroom for living. It is the simplest to permit.

The difference matters because a full ADU triggers utility, parking, and sometimes rental rules that a storage shed does not. For the distinctions in detail, see casita vs guest house vs ADU in Arizona and what is an ADU in Arizona.

What the Arizona ADU law requires

Arizona's ADU law forces larger cities to allow detached casitas on most single-family lots, with limits the city cannot exceed. A.R.S. 9-461.18 applies to municipalities with a population over 75,000. It requires those cities to permit "at least one attached and one detached accessory dwelling unit as a permitted use" on lots where single-family homes are allowed. The statute also caps how strict a city can be:

• Size. An ADU cannot be limited to less than 75 percent of the gross floor area of the main home or 1,000 square feet, whichever is less. So the city has to allow up to that size.
• Setbacks. A city cannot require rear or side setbacks greater than five feet from the property line for an ADU.
• No extra barriers. Cities cannot require added parking, demand the ADU match the main home's exterior, require a family relationship between occupants, or force public street improvements as a condition.

The law set a deadline of January 1, 2025 for cities to adopt compliant rules. If a city missed it, the statute says ADUs are allowed on all residentially zoned lots in that city without those limits. Smaller towns and unincorporated county land follow their own zoning, so a Carefree or Rio Verde Foothills lot may have different rules. Confirm your jurisdiction first using how to know if your lot is in the city or unincorporated county. For the full requirements, see Arizona ADU requirements and how many ADUs you can have per property.

Setbacks, lot coverage, and placement

Before you pick a spot for the casita, confirm your setbacks and lot coverage, because they decide where on the lot it can go. Setbacks are the minimum distances a building must sit from each property line. For an ADU in a covered city, the rear and side setback cannot be forced above five feet, which gives you real room to tuck a casita into a back corner. A detached garage or shop without a dwelling may face the standard accessory-structure setbacks instead.

Lot coverage is the share of your lot that all buildings can cover. Your house, garage, and the new casita together have to stay under the cap. On a generous lot in Cave Creek or Rio Verde, that is rarely a problem. On a tight Scottsdale infill lot, a large casita might push you over. Height limits apply too, and a casita usually has to stay single story or under the district's height cap. The placement also affects cost, since a casita far from the house means longer utility runs.

Running utilities to a detached casita

The biggest hidden cost of a detached casita is getting water, sewer, and power out to it. A casita with a kitchen and bathroom needs:

• Water. A new supply line from the house's service or the meter out to the casita.
• Sewer or septic. A connection to the city sewer, or, on a rural lot, capacity in your septic system. An existing septic field may not be sized for a second dwelling, which can mean an expansion and county health review.
• Electrical. A new circuit or subpanel run from the main panel, sized for the casita's load.
• HVAC. Its own heating and cooling, since Arizona heat makes a shared system from the main house impractical for a separate building.

These runs are trenched across the yard, so distance drives cost. A casita 60 feet from the house costs more to connect than one against the back wall. On rural lots in Rio Verde Foothills, water can be its own project. If the home is on hauled water or a shared well, adding a dwelling changes the demand. See building in Rio Verde Foothills water rules for that situation.

One detail trips up many owners: the existing electrical panel. A casita's kitchen, water heater, and air conditioning add real load, and an older 100-amp panel may not have the spare capacity to feed a second dwelling. The plan reviewer checks this. If the panel is full, you may need a panel upgrade before the casita can be served, which is a separate cost and permit. The same goes for septic. A field sized for a three-bedroom home will not automatically carry a fourth bedroom in a casita, and the county health department has to sign off on the added flow.

Permits and the build

Every casita and detached structure with utilities needs a building permit, pulled before construction starts. You submit plans to your city or county, usually through an online portal like Scottsdale's SPUR, and the plans go through review. The reviewer checks that the casita meets current building code, fits your setbacks and lot coverage, and connects its utilities correctly. Phoenix and other cities treat a detached dwelling as new residential construction, so the plan and inspection requirements mirror a small home.

Inspections follow the same staged path as any build: foundation, framing, rough electrical and plumbing, then final. A detached structure on expansive soils, common across Maricopa and Pinal County, needs a foundation engineered for soil movement, the same as the main house. The practical order is clear. Confirm your zoning and ADU rules, design within the size and setback limits, plan the utility runs, submit stamped plans, pull the permit, and build to the approved plans. For what a casita costs to build, see cost to build a casita in Arizona. Because city rules and adopted code editions vary, confirm the specifics with your local building department before you design.

The Jematell Homes approach

We take on additions, remodels, and teardown-rebuilds across the Valley. 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

• A.R.S. 9-461.18 (Accessory dwelling units; municipal requirements)
• City of Scottsdale: Accessory Dwelling Units (size standards)
• City of Phoenix: Residential Additions and Remodels
• City of Scottsdale: Permit Services