An office building's septic design flow is normally set from employee count or plumbing fixture units, and the wastewater itself is typically low-strength, closer to household sewage than a restaurant, unless the building adds a cafeteria or a heavily used break-room kitchen.
Occupant count and restroom fixture count, not bedrooms, are the base of the design flow calculation, and because most office wastewater is restroom and break-room use, it does not usually carry the high organic loading that food-service wastewater does. That keeps a typical office closer to the conventional end of commercial design.
An on-site cafeteria, a full commercial kitchen serving a company dining program, or heavy break-room dishwashing changes that picture and can push a building toward grease-interception and high-strength-waste considerations that a plain office would not otherwise face.
A building leased to multiple tenants can see its actual occupant load shift significantly as tenants turn over, which matters for a septic system in a way it does not for a sewer connection: an onsite system has a fixed approved design flow, and a materially denser tenant mix than what was originally permitted is a real compliance question, not just a leasing one.
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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Normally from employee or occupant count and restroom fixture units under your state's commercial table, unless a cafeteria or significant kitchen use is added.
Not usually. Office wastewater is typically lower-strength, closer to ordinary household sewage, unless the building has a substantial food-service component that adds grease and organic load.
It can, if occupant load rises meaningfully beyond what the system was originally approved for. That is worth checking against the property's recorded design before signing a denser lease.
Page last reviewed 2026-08-31.