We got a shed dormer; for the landing stairs, more house width would have been necessary. But that’s not possible for you, right...?
The thing with the subsequent extension... I don’t know.
The plasterer and screed layer strongly advised against it because of all the dirt. We then skipped screed in the attic because otherwise, underfloor heating would have been necessary right away. But we had the plaster done immediately, because such a small area would probably be quite unattractive to get a company to come at some point. Any necessary insulation between the rafters can’t be pushed in either, and then, so that the heat doesn’t hit through the pure concrete ceiling and the dust is better contained, we also laid some Styrofoam and OSB on the floor. The basement, on the other hand, is already completely finished, and I’m really glad about that—no idea if in 5 years I’d really feel like doing that work; for now, I want to have peace from such construction topics.