Frost line depth in Winnebago County, IA
How deep the ground freezes in Winnebago County, IA
51 in. Dig to 51 in, just below the frost. Nearest NOAA station: Forest City 2 Nne, 8.6 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.