okay thanks for the useful hints guys, I know now how to deal with this little issue. Will play around with room first then walls, wall first then rooms, recompute etc. to see which way suits me best. Thanks appreciate it

@Keet hmm that's strange, I can see the images even when looking up this post on my smartphone.
Yes, your assumption is correct. I was using "export all". I've tried that, select all and export selected, this works, as the "cut-out-ground" is then not exported.

@Puybaret, I've tried to move corners ...

Started to play around with sh3d 7.2 on MacOS and now on 7.3.
I've created a little 3d Model, created the rooms first, then double clicked to create the walls, everything looks good so far in sh3d. I've exported that (.obj) and imported it into Unreal Engine 5. When going through the building, I ...