Colorado has no statewide residential building code, so there is no state frost depth number at all; the city or county that issues your permit sets the frost depth, and the only construction code the state imposes everywhere is an energy code that local governments layer onto whatever building code they have already adopted.
Colorado leaves the frost line depth to the local building department, which is where the number for an address comes from.
State law requires local jurisdictions to adopt and enforce the Colorado Model Low Energy and Carbon Code upon updating any other building code. Under state law, jurisdictions can make any amendment to the energy code they deem appropriate for local conditions. ... Cities and counties with building codes must adopt this code, or a more stringent code, when adopting or updating any building code after July 1, 2026.Building Energy Codes Toolkit, Colorado Energy Office, 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 Colorado 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. Leadville is the coldest of the 3 cities on the record in Colorado.
1,797degree days
average freezing index a winter
Leadville Lake Co AP
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Leadville | Colorado | 1,797 | 2,790 | 266 |
| Grand Junction | Colorado | 370 | 1,260 | 134 |
| Denver | Colorado | 267 | 742 | 155 |
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 Colorado marked at 811 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 Connecticut at 42 inches, Michigan at 42 inches and Vermont at 5 feet. That is a reading of other states’ codes rather than a depth for an address in Colorado: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Colorado code read September 22, 2026 from Building Energy Codes Toolkit, Colorado Energy Office. 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.