Ground floor approx. 100 sqm, upper floor expandable (planned bathroom, 2 children's rooms, 1 storage room)

If the extension is not to become an illegal building, the planner who has to make it approvable from an insulation perspective will really have a hard time. Starting with the absence of an exterior wall in the attic, it won't be a walk in the park to make this upper extension tight to the thermal envelope.

Your idea of considering a batten doubling on the rafters, which is necessary solely because of the thickness of the insulation membrane, as the counter-batten is nonsense from the same stable as the ten-centimeter floor.

Don't you think it's possible to make the thermal envelope tight? Basically, the OP could just stuff everything behind the knee wall with insulation material. Where's the problem? The floor?

Where is the problem? The floor?

No, the ceiling slab should currently even define the boundary of the thermal envelope – although the stairwell doesn't quite seem to fit in there.

Don’t you think it’s possible to make the thermal envelope airtight?

Yes, it is possible – and roughly as elegantly as you suppose. But it will already become a "what comes when" puzzle when installing the knee walls, which could have been planned and decided more straightforwardly from the start to reach the goal.

To me, the whole rigmarole looks like the planner has postponed the attic just as long as the builder (who now wants to prepare it immediately). Both should have thought about the latter earlier. At least an enclosure of the full staircase to the provisional attic should have been included, better yet all pipes routed up immediately and the knee walls already finished (possibly except for the inner paneling to later finesse the electrical work). But only considering whether and where to place a tub in the attic after the roof is covered is practically exemplary planning botch.

Is the bathroom on the upper floor really necessary and essential?

Since there is a full bathroom downstairs, a small shower bathroom upstairs would be sufficient.

Due to the planning of an apartment for 2 on the ground floor and a normal upper floor of a family house above, you end up with 2 master bedrooms, 2 complete bathrooms, and mini rooms.

The house is too small for that.

Why is the roof not insulated upstairs now and included in the thermal envelope right away?

...straight-up exemplary planning botch.

Now with this statement you can stand at the back of this thread. I’m afraid the OP might run away from us if we just keep complaining. Solutions are needed.

Why isn’t the roof insulated at the top now and included in the thermal envelope right away?

Out of money? I would also consider going through with the roof insulation now. That way you can also show the "botched planner" his nonsense. Maybe he will lower the price a bit.

Wasn't much more expensive for us. Insulating the floor and staircase costs too, and then you rip it out again or it's unnecessary. Especially here the clock is ticking that it becomes unnecessary. With a child, money no longer

Why isn't the roof insulated at the top now and included directly in the thermal envelope?

I can only assume that it is not - based on the plans and photos alone, some things are not clearly evident here. Without a detailed cornice box closure, I would actually expect a ground floor as a closed envelope here, but then with a pull-down staircase or enclosed staircase.

Solutions need to be found.

I’m afraid that even with a SERIOUS L (ERSTNL) one cannot solve this yet.