@orenjiAi hence my first 'easy' solution of removing the ceiling.. it is how I do it with basegame only.
However, Xinqun made the good remark that the ceiling of the lower floor only shows up if you have completely 'exploded' (deleted, un-roomed) the room you are working with. This means the room where the stairs are in will not react as a room to weather, friezes, trims and all those fun things. Hence the more complicated workarounds needed if you wish to keep the space a proper room... That's why the frustration with the room based building popped up here too :)