Arizona has no statewide residential building code and publishes no frost depth number, so the required footing depth comes from the city or the county board of supervisors, and a county may leave rural or unclassified land out of its code entirely.
Arizona leaves the frost line depth to the local building department, which is where the number for an address comes from.
A. In any county that has adopted zoning pursuant to this chapter, the board of supervisors may adopt and enforce, for the unincorporated areas of the county so zoned, a building code and other related codes to regulate the quality, type of material and workmanship of all aspects of construction of buildings or structures, except that the board may authorize that areas zoned rural or unclassified may be exempt from the provisions of the code adopted.A.R.S. 11-861, Adoption of codes by reference, Arizona Revised Statutes, Arizona State Legislature, read September 22, 2026. Read it at the source
That makes the building department issuing the permit the office with the number, and it is a short call: the counter gives the depth for the address, and the same call settles whether a deck footing or a slab edge needs a footing inspection before the concrete goes in. What Arizona does publish is below, and it is worth having in hand first.
The air freezing index is the quantity the frost provisions of the model codes are written on: add up how far each day’s mean temperature fell below freezing over a winter, in degree days, and take the deepest run the winter managed. It measures how much cold the ground had to absorb, which is why it is the honest way to set one place beside another. Flagstaff is the coldest of the 3 cities on the record in Arizona.
293degree days
average freezing index a winter
Flagstaff Pulliam AP
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Flagstaff | Arizona | 293 | 883 | 205 |
| Phoenix | Arizona | 0 | No freezing winter on record | 5 |
| Tucson | Arizona | 0 | 5 | 14 |
Computed from NOAA National Centers for Environmental Information, Global Historical Climatology Network daily temperature records for each station, from 1951 onward. These are measurements of winter, not depths.
Only a handful of states answer with one number for the whole state, and they are the only codes that can be set beside each other directly. Ranked by the freezing index at their own reference cities, with Arizona marked at 98 degree days for comparison against what those codes settled on:
Left to right, the mildest winters to the hardest. The figure on the right is what that state wrote into its code.
The nearest by winter are North Carolina at 12 inches, Massachusetts at 4 feet and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Arizona: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Arizona code read September 22, 2026 from A.R.S. 11-861, Adoption of codes by reference, Arizona Revised Statutes, Arizona State Legislature. Codes are amended and a local amendment can be deeper than the state figure, so the building department issuing the permit has the number that binds. Every state is on the frost line depth hub.