A septic system is one of the most expensive components of a rural property and one of the least examined at purchase.
What a proper inspection includes
Locating the components — the tank, the distribution box, and the field — from records where they exist and by probing where they do not.
Opening the tank at both access lids, not just one.
Measuring the sludge and scum layers, which says how it has been maintained.
Inspecting the baffles or tees, inlet and outlet. A missing outlet baffle is common in older tanks and it means scum has been going to the field.
Checking the liquid level. Above the outlet invert means effluent is not leaving; below it means the tank is leaking.
Checking the effluent filter, where one exists.
A hydraulic load test — running a substantial volume of water into the system and observing whether the field accepts it.
Inspecting the ground over the field for wet spots, odour and differential grass growth.
Checking any pump, floats and alarm.
What a "visual inspection" gives you
Someone lifting a lid and looking in.
That confirms the tank exists and is not obviously collapsed. It says nothing about the drainfield, which is where systems fail and where the money is.
If a report does not mention the field and a load test, it did not evaluate the part that matters.
The dye test
Dye is flushed into the system and the ground over the field is watched for it surfacing.
It is a reasonable supplementary check and it is not sufficient on its own: a field can be near the end of its capacity without surfacing on the day of the test, particularly in dry weather.
Timing
The wet season is the honest test. A field at the limit of its capacity performs in August and surfaces in March.
Where the inspection has to happen in the dry season, the load test matters more, and so does the maintenance record.
The records to ask for
The permit and the as-built drawing, held by the county health department in most jurisdictions. It shows the tank size, the field size and its location, and the date.
Pumping records, which say whether it has been maintained.
The design flow, in bedrooms, which must match the house being sold. A four-bedroom house on a system permitted for three is a problem that transfers to the buyer.
The questions that matter
How old is the system, and what is the expected remaining life. A well-maintained conventional field commonly runs twenty to thirty years.
Is there a reserve area available and unencumbered, and is it protected? Without one, a failed field has nowhere to go.
Has the house been altered — bedrooms added, a garbage disposal fitted, a basement finished — since the permit?
What it costs against what it protects
An inspection is a few hundred dollars. A replacement system is a large multiple of that, and on a difficult site an alternative system is a larger multiple again.
It is the cheapest contingency on a rural purchase, and the one most often skipped because the house looked fine.
In many places it is mandatory
A number of states and counties require an inspection, and sometimes a pumping, at the point of transfer.
Where it is required, the standard is set by the jurisdiction. Where it is not, the buyer sets it — and asking for the full scope above rather than accepting a visual is entirely reasonable.
Who to hire
An inspector licensed or certified for septic evaluation in that jurisdiction, and ideally not the company that would perform the repairs they recommend.
A general home inspector's report typically excludes the septic system explicitly, which is worth reading rather than assuming.