Frost line depth in Mason County, MI
How deep the ground freezes in Mason County, MI
35 in. Dig to 36 in, just below the frost. Nearest NOAA station: Ludington 4 Se, 4.2 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.