Building a Casita or ADU in Phoenix? Plan the Electrical First

Casita electrical in Phoenix is one of the first things we'd sort out on any guest house or ADU project, before the slab is poured. A casita is a small house. It needs cooling that can handle a Phoenix July, hot water, lights, outlets and often a kitchenette and laundry. All of that has to come from somewhere, and in most cases that somewhere is a panel that was sized for the main house alone.

More homeowners are asking about this since the rules changed. Arizona's HB 2720, passed in 2024, requires cities with more than 75,000 people to allow accessory dwelling units on single-family lots, and Phoenix updated its zoning ordinance in late 2024 to comply. Phoenix even publishes free pre-approved plans for detached ADUs. The City of Phoenix ADU page has the details. Zoning, setbacks and HOA rules still apply, so check those first. Then check the electrical.

Casita electrical in Phoenix starts with a load calculation

The main question is not whether a casita can be wired. It always can. The question is whether the house's existing service has room for it. A load calculation adds up what the house draws today: one or two AC systems, range, dryer, water heater, pool equipment. Then it adds what the casita will draw. If there's room, the casita gets a breaker and a feeder. If there isn't, the service gets upgraded first, and it's much cheaper to learn that on paper than after the breaker keeps tripping on the first hot afternoon.

This is where a lot of Valley homes land in the middle. A 1970s or 1980s house with a 150A or older 200A service, a pool and a newer AC can be closer to full than the owner thinks. 200A vs. 400A: which service you need →

Subpanel from the house, or a second meter?

Most casitas are fed from the main house: a breaker in the main panel, a feeder run through the yard, and a subpanel inside or on the casita. It's one electric bill, one utility account, and the least paperwork. The trade-off is that the casita shares the house's capacity.

A separate meter gives the casita its own service and its own bill, which some owners want for a rental or a family member who pays their own way. It also means utility involvement, a second service entrance and its own set of requirements, and not every utility, city or property allows it. Whether you're on SRP or APS changes the process. We check that for your address before you commit to either option. How the SRP and APS process works →

Cooling and hot water set the size of the job

In Phoenix, cooling isn't optional in a space someone lives in. A ductless mini-split is the usual choice for a casita, and it needs its own dedicated circuit and a disconnect at the outdoor unit. Size the cooling for a small building sitting in full sun, not a mild climate.

Hot water is the sleeper. A small tank water heater is a modest load. An electric tankless unit can draw more than the rest of the casita combined, and it's the single choice most likely to push a feeder, or the whole service, up a size. Add a kitchenette range, a stacked washer and dryer, and maybe an EV charger in the casita's parking spot, and the numbers add up quickly. What to know before adding an EV charger →

The trench: route, conduit and distance

A detached casita is usually fed underground. Before anyone digs, call Arizona 811 to have the buried utility lines marked. Backyards in the Valley often hide pool plumbing, irrigation lines, gas and old low-voltage runs, and hitting any of them costs a day.

Burial depth depends on the wiring method and where the trench runs, and the inspector sees the trench open before it's backfilled. Distance matters too. A casita at the back of a deep lot may need larger wire than the breaker size alone suggests, so voltage drop doesn't leave the casita's AC starting weak. If you're pouring a patio or putting in landscaping between the house and the casita, run the conduit first, and consider a spare conduit for internet or future low-voltage.

Inside the casita

• A disconnect and grounding at the building. A detached structure gets its own means to shut the power off, and its own grounding. The neutral and ground are kept separate in the subpanel.
• A subpanel with spare spaces. Leave room for what gets added later: a second mini-split head, a heater for the bathroom, a charger.
• Shock and fire protection. GFCI protection in the kitchenette, bathroom, laundry and outside, and arc-fault protection where it's required for living space.
• Smoke alarms, and carbon monoxide alarms if there's any gas appliance.
• Exterior lighting and outlets on the path between the house and the casita, rated for sun and monsoon.

Garage conversions and attached ADUs

Not every ADU is a new building. Converting a garage or adding an attached suite often means tracing wiring that was never meant for a bedroom, relocating a panel that suddenly sits inside living space, and adding circuits for cooling. It's also the most common place we find unpermitted work from a past owner. We check what's there before the drywall goes up.

Casitas across the Valley

Phoenix, Mesa, Chandler, Gilbert and Scottsdale are all over the 75,000 mark, so the state ADU law applies to all of them, but each city runs its own permits and inspections. In newer neighborhoods in Gilbert and the East Valley, casitas often go on lots whose builder-installed panel has limited open spaces. In older central Phoenix ranch homes, the service itself is often the limit. Either way, the answer comes from the load calculation. What a panel upgrade costs in Phoenix →

Planning a casita or ADU?

Bring us in while the plans are still being drawn. We'll run the load calculation, tell you whether a subpanel from the house will work or the service needs an upgrade, and give you a free written estimate you can hand to your builder. We work direct to homeowners and as a subcontractor to general contractors across Phoenix, Scottsdale, Paradise Valley and the East Valley. B and D Electric LLC, Arizona ROC 361086, panel and service upgrades. Call (480) 828-1181 or request an estimate.