Frost line depth in Douglas County, MO
How deep the ground freezes in Douglas County, MO
18 in. Dig to 19 in, just below the frost. Nearest NOAA station: Wasola 5n, 7.3 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.