disappearing floor, without crystal clear or the marble flooring

Hello @Flyingmops !  First of all, that is a very interesting build.  I love the design!  I'd like to try and help if I can but I'm not sure of what I'm looking at in your screenshot. Should there be flooring within the space of what looks like exterior trim?  If so, it may be that the floor was deleted somehow.  When you click a wall in the room it should highlight the whole room and there should be a yellow outline around the perimeter.  Clicking on that should give you the option to "build floor".  Edit: If that is exterior trim I see, then clicking the trim should highlight the whole room as well.