@2. I think, now I've understood.
If I have to solve this problem I could use one of two solutions:
S1. Draw the exterior (and the interior structural) walls with their thickness and maximum needed height.
Then add how many levels you want at needed elevation.
Draw necessary rooms (not need to cover the entire level).
S2. Draw a big room as interior limit of building.
Add levels with rooms and furniture (machines).
Then surround it with walls.
For an existing building probably I will use first method.
I'm not a developer, and not a professional for designing houses.
Me either! Moreover, I'm not a programmer either.About IFC: While have another type of license not GPL probably someone could make a plugin to export/import this kind of file format.
My personal opinion: SH3D was made for peoples like me who don't have many skills or knowledge but want to dream at their home.
So, it appear we don't have a deal
but we cud remain friends. 
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A computer program does what you tell it to do, not what you want it to do. Murphy's Law
When all else fails, read the instructions. Murphy's Law
If you don't like "AS IS", DIY. Dorin's law
