I think, the whole room concept centered building is very bugged, and there are a few solutions to get around that. The use of spandrels, @Xinqun , is indeed a good idea, I need to check that out.
@DeanXeL , maybe that is a difference then, I have all addons. And I am making heavy use of friezes and trims, and cause cutting out the floor does have an impact of how a room is recognized by the game, I usually have to fight against friezes on the inside walls, where my staircases are. The last big staircase - I needed roundabout three hours to get it to my likes. The rest of the house was easily done but a double staircase, that is only partially walled, in the entrance area with an open ceiling, combined with stairways into the attic and into the basement: I think, I understand, what Xinqun meant with the word "exploded" here...
That house was then the reason for me to document every single step there. And like I wrote, my description sounds much more complicated than the process actually is.