thanks for your link. I checked the video that was in the comments (ACAD dwg Import into SketchUp - YouTube) and it seems that this is solving my problem:
-select all
-intersect faces → with selection
I tried to reproduce, and it only happens when using the ‘explode’ on my floorplan. Maybe there is a better way to make the floorplan transparent so you can still see once you start drawing the walls…