About Your design:
1. Wile You already know how to export/import walls/objects make a sloping wall and import it in a different position (from vertical to horizontal) or use Shape Generator plugin.
2. I think the door in the stair case is dangerous for head or useless.
I suggest this:
or this:
3. Make more room for bathroom and put lavatory in it. I think there is their place.
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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
