Thanks for your feedback.
I was able to "correct" the file by importing it into MeshLab, exporting it in STL format, re-importing it and then exporting it in OBJ format again. I was then able to import the corrected OBJ file into Sweet Home 3D, and it works fine.
It's great you succeeded! Were you able to keep colors too?I can't discover how to move my virtual visitor sideways using the keyboard, none of the keys (e.g. arrow keys) seem to work....
You can't from the 3D view. If the focus is on the plan view, it's possible with the arrow keys if the virtual visitor selected and is parallel with borders.I don't know how to select a wall under a staircase, for example, without moving the staircase...
Right click in the plan view where the staircase is and select it in the Select object sub menu of the contextual menu that will appear (see also the end of this blog article).Also, I don't know how to insert an additional point (vertex?) in a room that has already been created/drawn...or how to delete unnecessary vertices...
You should use the Advanced editing plugin. See this tutorial written by Hans....and if it is possible to position the building on sloping ground - our planned new meditation centre that will be a refurbished and extended 3 storey house is on a sloping site.
This feature doesn't exist yet. You'll have to simulate it with triangles for example. See also this thread.I wonder if it is possible to create flat (horizontal) roof lights/windows in flat roofs/ceiling...
Not directly, but in a separate document, you can create a wall of the same size as your roof with a window, then export it to OBJ format and finally reimport it as a piece of furniture, changing its orientation at the second step of the importation wizard to make the wall oriented like the roof and not vertical.I can't find a flat panel ceiling light (fluorescent or LED) which I need to light some rooms -do you know if one exists?
Not in Sweet Home 3D library.One minor comment about the user interface, when I double click on an item (wall, window, etc) I want to open its dialogue box to inspect it or modify it, but sometimes I accidentally move the item on the plan, which I don't want. Maybe it would be possible for the program to automatically undo the accidental movement of an item caused by a double click?
It's difficult to guess what is an accident and what is not. 
I remember I tried to fix some similar issue some time ago. I'll try to check if I can improve this.
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Emmanuel Puybaret, Sweet Home 3D creator
