..not viewable with a new checkbox in the level dialog box. When a level is not viewable, the objects added to it are simply not displayed in the furniture list, the plan and the 3D view,

Very nice. Congratulations. So far nothing wrong, but then I need to figure out different things to do with this feature. For now it seems a very cool way of toggling alternative layouts, but the process is (ever so slightly) disturbed by the three step process for hiding and showing the alternatives.

Would it be easier to use if a tab checkbox, right-clicking (or something) the tab toggles visibility? Maybe a context menu on the tabs with "Add level", "Modify level" "Delete level" "Show/hide level". This would de-clutter the 2D plan menu and be easy to use (but I guess following your stringent menu thought and concern about some users not being used to context menus could be an argument against.) Just a thought.

And don't get me wrong. I really like the feature the way you implemented it.

ok