ᐅ Costs for a 2.5- or 2-Story Flat Roof Building

Hello,

there is a common idea that multiple floors with the same total area are cheaper. Does this also apply in the case of a flat roof and 2 to 2.5 floors?

So what is more cost-effective?
1) 2 floors, flat roof
2) 2.5 floors (the top half floor is a normal floor but with a maximum of half the floor area of a full story), flat roof

Both with the same living area.

Thank you very much
Daniel

Why should 2.5 stories be cheaper than 2?

I think the question of whether 1.5 stories with a gable roof are cheaper than two stories with a shallower roof makes more sense.