Measuring the size of an apartment - walls included or not

There is no legal definition whatsoever of how to measure a property in Belgium.

Normally, the figure you get is a rough estimate of total floor space measured off the plans. Unfinished areas (cellar, garage) count for 50%

I'm sure from when we bought our place we were just given the area of the amount of ground that the building stands on and of the whole plot.
From what I have heard and read, the only thing of legal significance is that there should be no deliberate intention to make a false representation - if a place is described as having 100m² and it only has 98 it's OK but if it only has 70 that's unacceptable. Similarly if all the rooms are individually described and they are all overstated, that, too, would be unacceptable.