If you've done a "proper roof" above a floorplan, you've probably covered everywhere (walltop and open space) with floor and/or bridge, except where you're providing inter-level access of course. And if you then decide to expand you'd obviously have to remove any floor that coincides with where you want to place a new wall[1], regardless of what it covers. With due diligence to avoid certain possible cave-in situations, of course, but that can be mitigated by being methodical.
If you're adding to a lower level before blanket-flooring it, then changing the floorplan is trivial. I sometimes reverse the rotational direction of a design, or even "inverse ziggurat" bits of it in the process, to increase usable space eith overhangs.
You really only need to know what you're doing in advance (ideally), in both cases, or if you're making it up as you go along there are plenty of clues along the way. It's no more complicated than trying to plan a fully congruent repeat. Which is difficult, in itself, but only to normal "until you understand the game's little quirks" DF difficulty[3]. IMO.
How to make this better with the new tileset, I leave open to answering by those in the core UI redesign team. I'm not sure anyone else has enough knowledge to narrow down the options to what is caterable-for.
[1] The argument that this is necessary[2] is, I expect, more Toady/ThreeToe country as an annexe to the UI reorganisation project.
[2] Prime alternatives being 1) to allow wall-building on built floors, 2) "conversion" of the floor material into wall-form or 3) "recovery" of the floor material at the same time as; all of them with pluses and minuses to their operation, if enabled as an option atop the current working, or maybe how the current thing is totally retooled to become.
[3] Like it being best to place all corner-pillars first, then (planned) door-to-door flooring, then expand the floor and the corners in. At least the way I tend to do it. With a close eye on the natural LIFO job ordering when it comes to 'servicing' such construction jobs. In fact, I'll block-designate walls and/or floors then cancel some and block designate those spaces in again (repeating to further sub-selections as necessary) to force the jobs into the order I require. But that's a discussiin for another thread, on another sub-board, even if the rest of this post is not.