Alright, I remember being able to do this before in Sims 4, but for some reason, it isn't working now. Consider the following... This is the third floor in a three-floor sample building. The balcon...
@eufl sounds more complicated than it has to be in this case I think ^_^ maybe the pictures help. The way I did it, you can still change the shape of the balcony afterwards if you so desire, i tried that. It will only add floor to the small part you separated (let's say you cut off a corner to make the balcony have diagonal corners). it will not add wall to the part that's integrated with the staircase and has no floor.