Costs for 2 or 2.5 story flat roof

Hello,

one often hears that more floors with the same area are cheaper. Does this also apply in the configuration of a flat roof and 2/2.5 floors?

So what is cheaper?
1) 2 floors, flat roof
2) 2.5 floors (the top, i.e. 0.5 floor is a normal floor, but with a maximum of half the "floor area"), flat roof

Each with the same living area.

Thank you very much
Daniel

Why should 2.5 stories be cheaper than 2? The question of whether 1.5 stories with a gable roof are cheaper than two stories with a flatter roof would seem more reasonable?