California publishes no frost depth of its own; the state adopts the model residential code into Title 24 and the frost line that governs a footing is whatever the city or county building department has set for that area, which in most of the state is shallow or nominal and in the Sierra and the northeast corner is not, so a reader in California asks the local building department before digging.
California leaves the frost line depth to the local building department, which is where the number for an address comes from.
All occupancies in California are subject to national model codes adopted into Title 24, and occupancies are further subject to amendments adopted by state agencies and ordinances implemented by local jurisdictions' governing bodies.Codes, California Building Standards Commission, 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 California 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. Tahoe City is the coldest of the 3 cities on the record in California.
252degree days
average freezing index a winter
Tahoe City
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Tahoe City | California | 252 | 769 | 203 |
| Fresno | California | 0 | 4 | 17 |
| Sacramento | California | 0 | 3 | 5 |
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 California marked at 84 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 California: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
California code read September 22, 2026 from Codes, California Building Standards Commission. 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.