If you connecting to a city sewer line the elevation of the existing line is what controls. THis is not the same as the frost level because of the heat normally carried by a sewer line contnts and the. - Not like preventing a footing in an unoccupied building from heaving.
It depends on where your located, up here in the northeast, drainage piping is required to be installed 4 feet below grade, water piping 5 feet below grade. The further south you go, the frost line is higher, so the drainage piping can usually be installed higher in the ground.