Setbacks are the minimum required distances between a septic tank or drainfield and things like wells, property lines, surface water, and building foundations. Every state and often the local health authority sets its own setback table, so exact distances vary; the categories they cover, and the reasoning behind them, are consistent across almost all of them.
Setbacks to wells and to surface water exist primarily to protect drinking water and waterways from effluent that has not yet been fully treated by the soil; these tend to be among the largest required distances, and are often larger still near a public or shared water supply. Setbacks to property lines exist so a system does not depend on or encroach on a neighboring lot. Setbacks to foundations protect both the structure (from moisture and settling near a drainfield) and the system (from soil disturbance and compaction near construction).
Slope and soil type can also affect required setbacks, since effluent moving through sloped ground can travel differently than on flat ground, and the state or local code accounts for that in its tables.
Every setback on a permitted system is recorded on the site plan filed with the county, which is the authoritative source for what applies to a specific property, since it reflects the code in force at the time of permitting rather than whatever the current code happens to be.
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Minimum distances to wells, property lines, surface water, and building foundations are the common categories, though exact distances are set by each state and often the local health authority.
Because effluent leaving the drainfield hasn't yet completed treatment, and these setbacks are meant to protect drinking water and waterways from it as it moves through the soil.
The site plan filed with the county at the time of permitting shows the setbacks that applied to that system, which is more reliable than looking up current code since requirements can change over time.
Guide last reviewed 2026-08-31.