A septic system isn't just sized — it has to be placed. Setback rules govern how far the tank and drain field must sit from wells, water, and structures. They vary by jurisdiction, but the ranges are fairly consistent.
Common setbacks
The water table
The drain field must keep a minimum vertical separation — typically 2 to 4 feet — between the bottom of the trenches and the seasonal high water table. That unsaturated soil is where final treatment happens. When the water table is too high, you can't meet the separation with a conventional field, which is exactly when mound and aerobic systems come in.
Why they exist
Every setback protects a path that pathogens could otherwise travel: into a drinking-water well, a neighbor's land, a stream, or the groundwater. The distances give soil enough room and time to filter effluent before it reaches anything sensitive.
Your county health department publishes the exact setback table and will site-check the layout before issuing a permit. Confirm distances before you stake anything.