Frost line depth in Yankton County, SD
How deep the ground freezes in Yankton County, SD
40 in. Dig to 41 in, just below the frost. Nearest NOAA station: Yankton 2 E, 9 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.