What an ADU Involves in Arizona
What counts as an ADU
An ADU is a complete living unit with its own kitchen, bathroom, sleeping area and entrance, on the same lot as a primary home. It can be detached, like a backyard casita; attached, like a suite added to the side of the house with a separate door; or a conversion of existing space such as a garage. Because it is a dwelling rather than a bonus room, it is held to the full residential building code for a home.
What Arizona and your city allow
Arizona changed the rules in 2024. State law now requires larger cities to permit ADUs on single-family lots, which opened the door in much of the Phoenix metro. Each city still writes its own standards for size, height, setbacks, parking and design, and smaller towns and county areas set their own rules, so the first step is checking what applies to your specific parcel. If you are in an HOA, its design review applies on top of the city's rules.
- Maximum ADU size, often tied to lot size or the primary home
- Setbacks from property lines and separation from the main house
- Height limits, which matter for two-story or above-garage units
- Parking and access requirements
- Whether the unit may be rented, and for how long
Checking the lot
Before design starts we confirm where a unit can physically and legally go: lot coverage after the ADU is added, utility easements, the location of the sewer line and septic if there is one, drainage, and how materials and equipment will reach the back of the lot. On a tight lot the access route alone can decide whether a detached unit is practical.
Designing a small, complete home
Good ADU design is about making a small footprint live well: a kitchen that actually works, a full bathroom, storage, natural light and privacy from the main house. The exterior should match the primary home's roof lines, stucco and colors so the property reads as one. Our in-house designers draw the unit with cost feedback as they go, so the plan you approve is already priced.
Utilities are the hidden scope
A detached ADU needs water, sewer, electricity and cooling run to a new building. That can mean trenching across the yard, a larger electrical service or a separate panel, a sewer connection or ejector pump, and its own air conditioning. Whether the unit shares the main house's meters or gets its own depends on the city and the utility. These items are often the difference between a rough guess and a real estimate, which is why we look at them on site.
- Water and sewer routing from the main house or street
- Electrical service capacity and a dedicated panel for the unit
- Heating and cooling sized for the unit, usually a mini-split or small split system
- Gas, if the kitchen or water heater will use it
- Internet and any separate metering the city requires
Foundation, structure and fire separation
A detached unit sits on its own engineered slab, designed for the soil on the lot, with footings, under-slab plumbing and the slab itself poured and finished before framing starts. Framing, roofing, windows and doors follow, then rough plumbing, electrical and mechanical. An attached ADU or garage conversion also has to meet fire-separation requirements between the unit and the main house, which affects walls, ceilings and doors.
Permits and inspections
An ADU is permitted as a dwelling, so the city reviews architectural, structural, plumbing, electrical and mechanical drawings, and inspects in stages from footings through final. Some cities also require a separate address or utility application for the unit. We prepare the submittal, respond to plan review and schedule every inspection.
What drives the cost
Size matters less than people expect. The big drivers are the utilities run to the unit, the foundation and site work, a full kitchen and bathroom in a small space, any electrical service upgrade, and the finish level. A garage conversion avoids a new foundation but adds insulation, fire separation and often a new garage door replacement with a wall and windows. The only way to a real number is a site visit.
The steps, in order
Here is how an ADU moves from idea to keys with All American Builders.
- Site visit: lot, utilities, access, what the city and any HOA allow
- Design and engineering with cost feedback; HOA submittal if required
- Itemized proposal tied to the drawings
- Permit submittal and plan review
- Construction: site work and utilities, foundation, framing, roof, rough-ins, inspections, insulation, drywall, kitchen and bath, finishes
- Final inspection, walkthrough, punch list and handover