>>11981051
>There's no place where a room would be directly under another room. In other words, any given Z coordinate belongs to no more than exactly one point on a traversible (X;Y) surface.

This is not true at all. Even the first level of the game has 2 rooms above one another (the cinema projection room above the corridor), as well as the spiral staircase.
In Build you can stack as many floors as you want, the only limit is that you can't see two stacked sectors at the same time (and this has nothing to do with the definition of 3D)