Which expansion stage for the attic as an expansion reserve?

Just make sure to do the plastering on the gable sides right away. Otherwise, you'll regret it later.

Can't you have the underfloor heating installed now? It doesn't have to be connected yet. Having two systems side by side could be detrimental to the efficiency of the underfloor heating later on.

How are you handling the stairs? A hatch first or already a full staircase? If the latter: Then I would consider the floor plan on the upper floor when planning the railing so that it isn't too short or too long later on.

and insulate the ceiling below

What is meant by this?

If you put the insulation ON the screed, you again don’t have a “proper floor.” If you put it under the screed on the concrete ceiling, the thickness is not sufficient to meet the energy saving regulations, and below the concrete ceiling in a suspended ceiling it becomes more than challenging in terms of building physics.

Once the screed is laid, you can/should also install the underfloor heating directly. For a 40m² attic, that will hardly cost an additional €1500. The “conversion” of the heating system alone to be able to operate radiators on it (e.g. mixer) will cost you about the same amount. The pipes and radiators themselves will presumably cost another similar amount, and of course, you will have permanently higher heating costs afterward. In short: financially, that would be gross foolishness.

In the end, you “save” with this action the additional costs that arise when insulating the roof instead of the ceiling (slightly larger surface). But if you consider that the insulation must be additionally “clad/covered” both on the screed and below the ceiling, while it could initially remain naked (with vapor barrier) in the roof slope (drywall later after moving in, done yourself), I simply don’t see any potential savings.

Can't you have the underfloor heating installed now? It doesn't have to be connected. Having two systems next to each other could later be detrimental to the effectiveness of the underfloor heating.

How are you handling the stairs? First a hatch or already a full staircase? If the latter: Then I would take the floor plan of the upper floor into account for the railing right away, so that the railing is not too short or too long later

It is possible, but a potential expansion (if it happens at all) is still far in the future. Also, it’s a bit of a budget issue slowly.

It will be a proper staircase since we want to use the attic as a basement replacement, storage space, and hobby room (e.g. rehearsal room, etc.).

What does that mean? If you put the insulation ON the screed, you again don’t have a "real floor". If you put it under the screed on the concrete slab, the thickness is not enough for the energy saving ordinance

Thanks for the hint. That was how the general contractor intended it. However, a thicker insulation than usual. I will inquire.

Later walls will then stand on the screed, which is not so good acoustically.