The proper depth to dig and install a water line depends upon your location. A water line needs to be deeper in colder climates. Local building codes will provide information on the proper water line depth for your area.
A water line must be installed below the frost line to prevent freezing. The frost line depth can vary between different counties within a state.
The building code for the State of Maryland requires that a water line be installed at least 36 inches beneath the ground. The minimum water line depth ranges from 36 inches to 60 inches in Indiana, but varies between counties.
The building codes in Placer County, California require that a water line be installed at least 12 inches beneath the ground. The measurement is taken from the top of the water line.