12cm step from hallway to basement rooms

Yes, they are standard interior walls. We will make the doors ourselves later.

But then the hallway is not inside the thermal envelope but simply a huge thermal bridge, not to say a gap, in the thermal envelope. I rather suspect that your general contractor somehow made a mistake in the planning (your reference to the faulty Gebäudeenergiegesetz calculation, although as described that also makes no sense, because as a residential basement perimeter insulation would of course have been necessary). Is the basement ceiling insulated now?

There is no insulation of the basement ceiling either.

The only insulation currently present in the basement would be an 8cm perimeter insulation on the exterior basement walls against the earth as well as the EPS boards under the screed in the basement hallway.

Was that approved like this? Is it already being built like this? I would have an expert take another look at it, that sounds completely confusing and not compliant with the [Gebäudeenergiegesetz].

How can anyone plan such nonsense?!

I don’t know how far along your construction project is, but if I were you, I would fully include the basement in the thermal envelope and that’s that. Then the perimeter insulation on the basement walls is increased to 12cm and the floor is insulated, and you save yourself all the trouble with insulation on the inside (ceiling, walls, and doors to the hallway).

For comparison, XPS at 12 instead of 8cm costs only 5 euros more per square meter. The labor for installation is the same. Inside, it balances out; whether the insulation is done on the ceiling or under the screed doesn’t matter cost-wise.

Huge advantage: You can use the basement much more flexibly.

And, how did it continue here?