Frost line depth in Sierra County, CA
How deep the ground freezes in Sierra County, CA
14 in. Dig to 14 in, just below the frost. Nearest NOAA station: Bowman Dam, 11.1 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.