Frost line depth in Clark County, ID
How deep the ground freezes in Clark County, ID
42 in. Dig to 43 in, just below the frost. Nearest NOAA station: Small 6n, 2.7 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.