Illinois publishes no statewide frost line depth: residential construction follows whatever code the city or county has adopted, or the current International Residential Code where none has been adopted, and the frost line depth in that code is filled in by the local building department, though the state plumbing code does set a hard floor of 36 inches for a buried water service line.
Illinois leaves the frost line depth to the local building department, which is where the number for an address comes from.
Until then, units of local government such as cities and counties can adopt codes of their choice. The best and most accurate answers to building code questions must be answered by your city, village, or county code official.Building Codes and Regulations, Capital Development Board, 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 Illinois does publish is below, and it is worth having in hand first.
A frost line depth turns into two separate requirements in most codes: how far a footing has to bear below grade, and how far a buried water service has to sit below the same line. Illinois writes one of them out.
A water service pipe is buried at least 36 inches deep, or deeper if the local frost penetration is deeper than that.
The minimum depth for any water service pipe shall be at least 36 inches or the maximum frost penetration of the local area, whichever is of greater depth.77 Ill. Adm. Code 890, Illinois Plumbing Code, Section 890.1150 Water Service Pipe Installation, Illinois Department of Public Health, read September 22, 2026. Read it at the source
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. Chicago is the coldest of the 3 cities on the record in Illinois.
660degree days
average freezing index a winter
Chicago O'Hare Intl AP
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Chicago | Illinois | 660 | 1,644 | 125 |
| Springfield | Illinois | 449 | 1,240 | 114 |
| Carbondale | Illinois | 196 | 678 | 104 |
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 Illinois marked at 435 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 Massachusetts at 4 feet, Connecticut at 42 inches and North Carolina at 12 inches. That is a reading of other states’ codes rather than a depth for an address in Illinois: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Illinois code read September 22, 2026 from Building Codes and Regulations, Capital Development Board. 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.