The basic rule is the ownership share, which is typically proportional to the floor area of the unit. If a 100 square metre flat has twice the ownership share of a 50 square metre one, its common cost will in principle be twice as high. This applies to every expense that cannot be clearly tied to individual consumption: caretaker wages, cleaning of common areas, insurance, the maintenance fund.
Exceptions are services where the law or practice requires accounting by individual consumption, typically heating and hot water, where Government Decree 676/2023 and related rules require consumption to be settled by individual metering where meters exist. Where there is no individual metering, heating costs are also split by ownership share or by a base defined in the house rules, such as heated cubic metres.
There is a third category: the house rules may set a different allocation key for certain costs if the general meeting so decides, for example lift costs charged only to floors above the lift entrance, or a ground floor shop paying for its own consumption via a separate water meter. Such deviations must always be recorded in the house rules, not decided case by case.
In practice most disputes arise because owners do not know the exact allocation key set out in their own house rules and assume everyone pays the same. A transparent monthly statement showing the ownership share and the resulting amount goes a long way to maintaining trust.
If you are unsure what your building's house rules say about allocation, Merbo's bookkeeping team is happy to walk you through the calculation of your own flat's common cost item by item.