A conventional onsite wastewater system disposes of treated effluent to the soil on the same property; once a system's design flow grows large enough, or once it discharges to surface water rather than to the soil, it can move out of standard onsite septic permitting and into a treatment-plant-style discharge permit, potentially including a Clean Water Act NPDES permit, and where that line sits is a state-specific determination.
Scale is one path: many states define a size threshold, often described in terms of design flow, above which a system is treated as a decentralized or community wastewater treatment facility rather than a standard onsite system, with correspondingly more rigorous engineering, permitting and monitoring requirements. Discharge type is the other path: a system built to discharge treated effluent to surface water, a stream, a ditch, a water body, rather than dispersing it to the soil, generally falls under the Clean Water Act's National Pollutant Discharge Elimination System (NPDES) program, which is a federal permitting framework typically administered by the state, and that is a fundamentally different permitting track from a soil-dispersal onsite system.
A project can also end up needing to connect to a public sewer instead of building any onsite treatment at all, where a state or local program simply does not allow a system of the project's size or discharge type to be permitted as onsite.
Finding out which track a project falls into, standard onsite, large/community onsite, NPDES discharge, or mandatory sewer connection, early in design affects which permitting authority to work with, what kind of engineering is required, and how long the process realistically takes. This is worth confirming directly with the state's onsite wastewater program and, where surface discharge is being considered, its water-quality permitting program, rather than assuming a conventional septic permit will cover it.
Find your state's septic regulator and permit sequence.
A new use, an addition, a remodel, a change of tenant, gets compared against what a commercial property is already permitted for. Where a recorded septic file exists for the property, it typically shows the tank size, the treatment components, the drainfield or dispersal area, and the design flow the system was approved for, which is the starting point for figuring out whether a change fits inside the existing approval or needs an upgrade. We publish where county septic records are kept and how many we hold, we list local septic companies by county, and we generate a free, code-referenced septic design for a parcel to use as a starting point.
Find septic records by county Find a local septic company Start a free design
Generally when a system is designed to discharge treated effluent to surface water rather than disperse it to the soil. That moves it under the Clean Water Act's NPDES program, typically administered by the state, rather than the standard onsite septic permitting track.
Many states set a design-flow threshold above which a system is treated as a decentralized or community wastewater facility with more rigorous requirements than a standard onsite system. The specific threshold is state-specific.
Yes, in some cases a state or local program will not permit a project of a given size or discharge type as onsite at all, requiring a public sewer connection instead.
Page last reviewed 2026-08-31.