A restaurant's design flow is calculated from seat count or plumbing fixture units under your state's commercial table, not bedroom count, and food-service wastewater almost always has to pass through a grease interceptor before it reaches the septic tank at all.
Two things separate a restaurant from a house: the wastewater carries fats, oils and grease (FOG) from cooking and dishwashing that a residential system was never designed to absorb, and the flow is driven by covers served and kitchen fixture count rather than by how many people sleep on the property. Both change the design math and both are handled through state tables, not a rule of thumb.
FOG that reaches a drainfield coats the soil and stops it from absorbing effluent, which is a slow, expensive failure. That is why a grease interceptor sized to the kitchen's fixtures sits between the kitchen drains and the septic tank, intercepting fat and solids before they ever enter the system that serves the rest of the building.
A retail space or office converting to a restaurant is the single most common way a commercial property runs into its septic system's limits, because the prior use was never designed for food-service flow or grease. The permitting authority will generally want to see the property's approved design flow and compare it against what the new kitchen needs before a change-of-use permit is issued.
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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From seat count or plumbing fixture units under your state's commercial onsite wastewater table, not bedroom count. The exact figures are state-specific; see your state's rules page for the regulator that publishes them.
Almost always yes. Grease that reaches a drainfield clogs the soil, so kitchen wastewater is intercepted before the tank, separately from the sanitary flow from restrooms.
Only if the existing approved design flow and grease capacity cover the new kitchen; that comparison is exactly what a change-of-use permit reviews, and it starts from the property's recorded design.
Page last reviewed 2026-08-31.