Frost line depth in Warren County, NJ
How deep the ground freezes in Warren County, NJ
24 in. Dig to 25 in, just below the frost. Nearest NOAA station: Belvidere Brg, 4.2 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.