Why a cesspit is different
A septic tank separates and discharges; a sewage treatment plant treats and discharges. A cesspit does neither. It is a watertight holding tank with no outflow at all, which is why it is used where ground conditions make a discharge impossible.
Because nothing leaves except by tanker, the volume to be removed is the whole of the household's waste water rather than the settled solids. That is the difference between emptying once or twice a year and emptying every few weeks.
The arithmetic, and why it is worth doing
The Lexsure wording below sets out the rule of thumb conveyancers use: an allowance per person per day, multiplied up to a monthly figure for a household. Applied to a family, the monthly volume is substantial.
Do that calculation for your household and for the capacity of this tank, and then ask a local contractor what they charge per emptying. The result is a running cost in the same order as a significant utility bill, and it is one nobody mentions when the house is being sold as having “private drainage”.
A tank sized for the couple who have lived there for thirty years may need emptying far more often once a family moves in.
What to establish before exchange
When the cesspit was installed, built, checked, repaired or rebuilt. Its estimated capacity. How often it has actually required emptying, and what that has cost — the seller's invoices are the best evidence available and are worth asking for.
Whether there is a maintenance notice displayed in the building, which building regulations require for a cesspit and whose absence is itself informative.
And have the system surveyed. A leaking cesspit is a pollution problem as well as a repair bill, and a watertight tank is the entire specification.