OSB panels for the attic "required", yet extra charge?

It "could" very well be that is correct in his assumption, but also that the BU simply confused the BV

That could very well be.

Once that is finally clarified, the OP can decide whether he now wants to have OSB there for 1000 euros (or: with cold roofs better not take that) or later carry tongue-and-groove boards up himself.

I like to repeat myself: Kaltdach is not what you think it is

Didn't sound like how you used it here in the context - it is not an uninsulated attic!

Moreover: With your roof windows, it becomes quite difficult to correctly construct a cold roof.

It didn't sound like how you used it here in the context - it is not an uninsulated attic!

Also: With your roof windows, it is quite difficult to correctly create a cold roof.

The cold roof in my case only exists in the area of the knee wall.

Otherwise, the roof itself is insulated as well. In the area of the knee wall ("attic") the upper floor ceiling.

That's why the double casement window is also not a problem. Otherwise, you are right: It wouldn't have been possible to build it that way.

And crystal clear and definitely: I have an *UNINSULATED* attic. Conditions there are always approximately outside conditions. There are even ventilation bricks in it.

Best regards
Thorsten