Frost line depth in Jefferson County, WI
How deep the ground freezes in Jefferson County, WI
41 in. Dig to 42 in, just below the frost. Nearest NOAA station: Jefferson Wwtp, 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.