Frost line depth in Yakima County, WA
How deep the ground freezes in Yakima County, WA
15 in. Dig to 15 in, just below the frost. Nearest NOAA station: Yakima #2, 12.1 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.