Don't worry for your English, your explanations are clear.
But I don't think elevating walls is a good solution, or said an other way, I'm sure there's a much better solution.
If you really want to elevate walls, you can already do it: use boxes as walls and it will do the same. I'll be glad to read whether this solution satisfied you or disappointed you once you will have tried with a real world example (meaning with furniture placed in rooms at upper level).
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Emmanuel Puybaret, Sweet Home 3D creator
