@DeanXeL , I'm not sure I understand the point of the second step in your process. Why not just push and pull on the hole you just made? Won't a platform, if it is brought down to the same level as the floor around it, just meld with the floor the next time you go to look at it? If the objective is to obtain the three squares over the stair for the auto-delete headroom thing to work (without having a fence or spandrel to define it) then, well, I can't help but feel that that's taking advantage of a bug that may or may not disappear one day and/or cause issues further down the road.
That thing about having to leave the three squares of floor over the stairs so the game can have a "ceiling to remove" is a new bug introduced with the platforms patch back in November. It's ruined a lot of old builds, including several premade builds that come with the game and its expansions, and they're still working on a fix for it.
@eufl , what I mean by an "exploded room" is a space made up of discrete, independent, freestanding walls. Simple test: click on a wall. Does it select the whole room, including the floor and all the other walls attached to it? Then it's a proper room. Does it only select the one wall, and can you move the wall around without affecting any other part of the room? Then it's an exploded room.
Why do rooms explode? Properly speaking, a room needs to have its shape defined either by the floor or by walls/fences/spandrels. If a "room" has no floor to define its shape, and if it is also not fully enclosed (ie, missing a wall/fence/spandrel so that one part of it is completely open) then its shape can really be anything. This causes the room to explode: the walls disconnect from each other and the ceiling disappears.
I know of at least one way around this, creating a room with no floor and at least one missing wall and somehow not having it explode. Apparently, once you've made your "room with no floor", you can drop a smaller floor into it, and it works. But, as I said earlier, I feel like it's taking advantage of a bug, and I don't trust it.