Frost line depth in Mendocino County, CA
How deep the ground freezes in Mendocino County, CA
2 in. Dig to 12 in, the 12 in code minimum. Nearest NOAA station: Willits 1 Ne, 5.4 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.