Frost line depth in Costilla County, CO
How deep the ground freezes in Costilla County, CO
38 in. Dig to 39 in, just below the frost. Nearest NOAA station: San Luis 1 S, 5.7 mi.
Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.