How do I keep an ADU cool in the Phoenix heat?

Start before construction: orient the casita to limit west-facing glass, shade windows and doors, use a light, reflective roof, insulate well above a garage's usual level, and size cooling for the real load. In 2024 Phoenix logged 70 days at or above 110°F, and a small building has little thermal mass to ride that out.

How hot is the design target?

Hotter than it used to be. KJZZ reported that 2024 was Phoenix's hottest year on record, with an average temperature of 78.6°F, 70 days at or above 110°F against a typical 21, and a record 113 straight days at 100°F or more. Design for those summers, not an average one.

What design choices matter most?

• Orientation. Put the long walls north and south, and keep west-facing glass small. Afternoon sun on a west wall is the hardest load to cool.
• Shade. Deep overhangs, a porch or a ramada over the entry, and exterior shade screens on east and west windows.
• Roof. A light-colored, reflective roof surface and a well-ventilated or insulated roof assembly.
• Windows. Low solar heat gain glass, and fewer, better-placed windows rather than many.
• Air sealing. Small buildings lose a lot through gaps at the slab, the top plate and penetrations.

What cooling system fits a casita?

For most studios and one-bedrooms, a ductless mini-split heat pump is the common choice: one outdoor unit, one or two indoor heads, and separate control from the main house. A 2026 Arizona cost guide budgets $6,000 to $14,000 for desert-rated HVAC on a casita. We size the system with a load calculation, not by square footage alone.

Is a garage conversion harder to keep cool?

Usually. Garages are built with little insulation and a large door opening, and many face the street with full sun. A conversion needs new wall and ceiling insulation, a sealed slab edge and a new wall where the door was. See garage conversions.

What about the monsoon?

Summer storms bring wind-driven rain and dust. We detail roof edges, flashing and the joint where an attached ADU meets the main roof with that in mind, and grade the site so water moves away from the slab.

Which codes apply?

The City of Phoenix adopted the 2024 International Codes, effective August 1, 2025. Scottsdale, Tempe, Glendale and Paradise Valley each set their own editions. The code is a minimum; in this climate it is worth asking what a little more insulation or better glass would cost.