Frost line depth in Nicollet County, MN
How deep the ground freezes in Nicollet County, MN
48 in. Dig to 49 in, just below the frost. Nearest NOAA station: New Ulm 3se, 9.2 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.