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You have the plans. Maybe it is a mother-in-law casita off the back patio, maybe it is turning the third garage bay into a rental, maybe it is a 600 square foot backyard unit you have been sketching since the law changed. The builder has given you a number. Then somebody asks the question that stalls the whole thing: is the panel on the house going to carry it?
That question has a real answer, and it is a lot cheaper to get before the slab is poured than after. Here is how the electrical side of an Arizona casita actually works, in the order you will run into it.
Arizona rewrote the rules. House Bill 2720, the bill most people call the Casita Bill, took effect January 1, 2025 and required every Arizona city over 75,000 residents to allow accessory dwelling units on single-family lots. That covers Phoenix, Mesa, Scottsdale, Chandler, Gilbert, Tempe, and most of the rest of the Valley. House Bill 2928 extended nearly the same framework to counties starting January 1, 2026, which pulled in unincorporated Maricopa County and the smaller towns around us.
So the zoning fight mostly went away. What did not go away is physics. Your house was wired for a house, not for a house plus a second dwelling with its own kitchen, its own laundry, and its own cooling.
Everything downstream depends on this. A casita is not one more circuit, it is a block of load: a mini-split or heat pump, a range or cooktop, a water heater, a dryer, a dishwasher, lighting, and receptacles that somebody is going to run a space heater and a hair dryer on at the same time.
The way that gets answered is a load calculation, not a guess. We add up the connected load of the house as it sits, add the casita as designed, and compare it against the rating of your service. A few patterns show up over and over in the Valley:
If the service does need to come up, that is a like-for-like 200 amp replacement in most cases, generally $5,200 to $6,500, or $6,400 to $8,750 when the meter base and panel are combined into a new all-in-one and the whole service gets replaced. Our panel upgrade cost breakdown walks through what moves that number, and the panels service page covers what the work involves.
The good news is that this is money you were probably going to spend eventually. A casita just forces the timeline.
The casita gets its own subpanel. That is not optional and it is not a place to economize, because every circuit inside the unit lands there and everything about the build after that point is downstream of how it was sized.
What drives the size:
Size it for what the unit will be in five years, not what is on the plan today. The feeder is the one part of this job nobody wants to redo.
Here is where casita quotes go sideways. The panel is on one side of the house. The casita is in the back corner of the lot. Between them is 80 feet of yard, and in a Valley backyard that 80 feet usually contains some combination of pavers, a pool, a pool equipment pad, a block wall, irrigation, and a slab patio.
The underground feeder runs in conduit at a required burial depth, and the path matters more than the distance. Straight across dirt is one price. Cutting a pool deck, boring under a driveway, or working around an existing pool and its bonding grid is another, and that is a conversation to have while the landscaping is already torn up rather than after it has been restored.
Two more things worth knowing about the feeder itself. First, long runs need to be sized for voltage drop, not just for amps. A 100 amp feeder that is correct at 40 feet can be undersized at 150 feet, and the symptom is not dramatic, it is lights that dim in the casita every time the AC starts. Second, feeders on a run like this are frequently multi-strand aluminum, which is completely normal and safe modern practice. That is not the same thing as the single strand aluminum branch wiring in some 1960s and 1970s Valley homes, which is the actual hazard people are thinking of.
Most casitas in the Valley run off the existing service on one meter. It is simpler, it is far less expensive, and it works fine when the unit is family, a guest house, or a long term rental where utilities are folded into the rent.
A second meter means a second service, which means coordination with APS or SRP and a much larger scope of work. It only makes sense when you genuinely need the tenant billed separately by the utility. Decide this early, because it changes the design of everything upstream.
Once the subpanel is set, a casita is wired like a small home, and code treats it like a dwelling unit rather than a shed with a bed in it. A typical build includes:
On the cooling side, most casitas in the Valley end up on a mini-split, and the electrical requirements for those are specific enough that we wrote them up separately in our guide to mini-split electrical requirements in Arizona. If you are still deciding what equipment to put in the unit, our sister company The Cooling Guy HVAC (Arizona ROC 364481) sizes these for casitas regularly, and getting the equipment picked before we rough in the wiring saves everybody a change order.
Converting a garage is usually the cheapest path to an ADU, and the electrical is usually the messiest part of it.
Garages are typically fed by one or two general purpose circuits that also serve the laundry, the exterior receptacles, and sometimes half the back of the house. The second that space becomes a dwelling unit, that wiring no longer covers it. You need dedicated circuits, correct receptacle spacing, GFCI and arc fault protection, and in most cases a proper subpanel rather than trying to extend what is there.
The other thing we find in garage conversions is existing work somebody did years ago without an electrician: an added receptacle on a lighting circuit, a 240 volt outlet for a compressor, a subpanel fed with the wrong conductors. All of it has to be sorted out before drywall goes up, which is one more reason to have the electrical walked before the framer starts.
Earlier than most people do. The right sequence looks like this:
The projects that go badly are almost always the ones where the electrical got treated as a finish trade instead of the thing the whole schedule hangs on.
Call us if you are pricing a casita or ADU and need to know whether your service can carry it, if a builder has told you the panel needs to be upgraded and you want that verified and quoted independently, if you are converting a garage and want the existing wiring evaluated before the work starts, or if you already have a unit going up and need the feeder, subpanel, and interior wiring handled by an electrician rather than a general crew.
We would rather look at your panel for free now than get called out after somebody buried the wrong conduit under a new paver patio.
At The Wire Guy Electric (Arizona ROC 365306), we do this work as a family-owned electrical contractor, which means the person who sizes your feeder is the same outfit that shows up at trim out.
We serve Mesa, Scottsdale, Gilbert, Chandler, Tempe, Queen Creek, Paradise Valley, and Fountain Hills. Request a free estimate and we will tell you exactly what the electrical side of your casita takes.
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