Stairs again...

@Xinqun , I think, we do mean the same thing here. At least I hope so, and I hope, my pictures now can proof either me understanding you right, or not... Let´s see! :)

Now I am back with a room, that exploded (as I would describe that), and tried several ways but found no solution so far.
The problem here is the stairway from the basement to the ground floor. This one can only be placed from the basement. And as you can see, it leads through the foundation, using a sunken platform to cover up the non existent foundation walls inside a building:


The floor above is still pristine, there is a porch with a roof running around, the roof still appears only on the outside:



I want to place a stacked staircase, so I need an L-shaped stair in the corner. But I can´t place it, cause there is no ground underneath. That is cause of the basement stairs. And believe me: Neither the platform nor the stairs to the basement can be built, if a stair is erected above. I tried this very often, no chance. So I have to build the basement stairs first (picture is ground floor, the basement stairs are parked outside, the sunken platform is still there):


To place those stairs, I need a hole in the ceiling, then I can place the stairs from above. I can place an L-shaped wall there, this works but not a half wall neither a fence, the room explodes:


I tried your way to surround the place with a platform, @DeanXeL , and the room explodes. I tried my usual thing with making a room without walls and place it in L-shape, where I want the stairs - the room explodes. Whatever I do, the room seems to get opened, cause a structure touches an outer wall.
Then, and this was my last try or now: What would happen, if I don´t touch the walls but delete the floor as a whole? It´s this here, the room explodes again:


Placing a floor, a room without walls, anywhere in the room, shows better, how the roof appears on the inside again:


Any idea? I think, we´re talking about a really exiting problem. I see this as a bug, cause that should not happen. But I am sure, there must be a solution. But now I am the one, who is completely blind, I don´t see any solution at the moment...

Btw, this is another small farmhouse for my StrangerVille expansion project, built after pictures of a real old farmhouse in Nevada.