I just want to tell that your guide still works but you need to leave the window where you are editing after you delete the slabs I thought that you still could be in that window and click at finished editing… My way was toooooo crazy but it works sometimes but hard to succeed 
your way is much easier … take the credits back I dont deserve it but you do
I think that I will delete my “crazy guide” now but add on your guide that the player need to leave the building designer window after deleting the slabs and not only click on the ghost window when you are in edit and then click on the “ghost window” again… tired and crazy people like I was yesterday dont get it LOL
I wait for you to say if you want me to delete it or not or maybe you can flag it when you see it… maybe funny reading for other players
another thing… I used to play this game last year or 2014 and found tutorials on youtube about basements for the houses and edit rooms on exsisting buildings but now when I am looking for that tutorial I cant find them… do you know where they are?
EDIT !!! : I found it and it was not on youtube it was here Basement/cellar tool … I am tired again and only searched for a video tutorial on youtube and not screenshots… LOL