Frost line depth in Lassen County, CA
How deep the ground freezes in Lassen County, CA
17 in. Dig to 17 in, just below the frost. Nearest NOAA station: Susanville 2sw, 20.7 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.