A real inspection opens the tank, measures sludge and scum, checks baffles and the effluent filter, and evaluates whether the drain field accepts flow. A visual walkover without opening the tank tells you almost nothing.
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There is a wide quality range in what gets called a septic inspection, and the difference matters more than the fee. A thorough inspection opens the tank lids, measures the sludge layer on the bottom and the scum layer on top, checks that the inlet and outlet baffles are intact, pulls and examines the effluent filter if one is fitted, and runs water into the system to confirm the drain field accepts flow without surfacing. It also locates the components and compares what is in the ground to the county permit record, which is how undersized or unpermitted systems get caught. A dye test flushes tracer dye and looks for it at the surface or in nearby ditches over the following hours. Dye appearing is definitive evidence of failure. Dye not appearing is not proof of health, because a field can be near the end of its capacity and still absorb a modest test volume. Treat a clean dye test as one input rather than a pass. What a visual-only inspection tells you is whether anything is currently surfacing. That is worth very little on a house that has been vacant or lightly occupied before listing, which is common, because the system has not been loaded. On age: a conventional system typically lasts 20 to 40 years depending on soil, maintenance, and loading. A 35-year-old system that inspects clean is still a system with limited remaining life, and pricing that into the offer is reasonable. Ask for the pumping records and the permit file as part of due diligence. A system with a documented pumping history is a materially different risk than one with no record at all. In Greater Columbus and Central Ohio, systems on smaller older lots across Franklin, Delaware, Licking, and Fairfield counties are frequently at or past their original design life, and some predate county permitting records entirely. An inspection that cannot locate a permit record is itself telling you something useful about the system's age.