Many states require a septic design to carry a professional engineer (PE) stamp once the system's design flow, or in some states its physical scale, crosses a state-set threshold, and below that threshold a licensed septic designer or sanitarian may be sufficient; where exactly that line sits is set by each state's onsite wastewater code, not a fixed national number.
Smaller residential-scale systems are typically designed by a licensed septic system designer, installer or sanitarian working within a state's standard design rules. As design flow grows, whether from a large restaurant, a school, a multi-family property or a community system, states generally require the added engineering judgment of a licensed PE, particularly where the design departs from standard configurations or where system failure would have a larger public-health or environmental consequence.
Some states set the PE requirement by a specific flow number, others by whether the system uses an alternative or advanced treatment technology rather than a conventional tank-and-drainfield design, and others by whether the project falls into a defined 'large' or 'community' system category. All three approaches exist across different states.
Before assuming a design can be handled the same way a small residential system would be, it is worth confirming with the state or county permitting authority whether the specific facility type and expected design flow trigger a PE-stamp requirement, since that affects both who can be hired for the design and what the permit application has to include.
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.
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No. Many states allow a licensed septic designer or sanitarian to handle smaller commercial systems, reserving the PE requirement for systems above a state-set flow or scale threshold, or those using alternative treatment technology.
It varies by state: some set a specific design-flow number, some trigger it by use of alternative or advanced treatment technology instead of a conventional system, and some by whether the project is classified as a large or community system.
Check with the county or state onsite wastewater permitting authority for that specific facility type and design flow. See our septic rules pages by state for the regulator to contact.
Page last reviewed 2026-08-31.