Sims 4 game sees room as outside

@AlexvonAuen That staircase is CC, so I'd definitely try the build without it as a first possibility.

Hi Luthien,

Thanks for the tip / idea. Was also something I had been considering.

However, I have tried numerous possibilities - using both the CC staircase and a 'normal' base game staircase.
In both cases they both work just fine.....until I add the railing around the open area... and delete the railing (i.e., fence/wall) in front of the top stair so that the Sims can actually use the staircase. This seems to be the problem.
This automatically also deletes the portion of the ceiling directly above the staircase - and the game then sees this as 'outside'.
Have also tried JosieDeco's idea of placing a 'flat square' (ground) above the staircase to 'close' the ceiling...but this seems to be only an 'optical' solution, as the game still sees the area as outdoors.

Also found a thread here by CGrant56 
     https://answers.ea.com/t5/Game-and-Mod-CC-Issues/Open-floor-over-stairs-deletes-them-and-makes-roof-enter/m-p/11267269
and tried that....but also of no use in this case. The fence / wall in front of the top stair does not 'automatically delete' and when I do it manually, the ceiling opens.

Even tried deleting the entire room (i.e., hallway) around where the staircase should be (in case there was something wrong with the room itself) and rebuilt it again from scratch. Same results.......

Guess I'll just have to get used to the idea of my Sims wearing their shoes indoors and being messy.....

Thanks anyway.

Alex