Detached villa with "gable roof" or flat roof

that is a huge area! Do you have a concrete floor there?

No.
We covered the floor with OSB boards about a year after moving in. It is only designed as a cold roof (for storage). The ceiling of the rooms below is made of plasterboard. In between, there is the vapor barrier and insulation.

The size of the freely accessible space also depends on the roof pitch.
So probably would have a bit less space available, since his roof pitch is lower—unless he has planned a few centimeters of knee wall in the roof.

No.
We covered the floor extensively with OSB boards about a year after moving in. It is only designed as a cold roof (for storage). The ceiling of the rooms below is made of drywall. In between, there is the vapor barrier as well as the insulation.

The size of the freely walkable space also depends on the roof pitch.
So, probably would have a bit less space available, as his roof pitch is lower - unless he has planned a few more centimeters of knee wall in the roof.

30cm knee wall at 25 degrees. That’s worthwhile. The ceiling is concrete… but also initially as a cold roof.

Best regards, Steffen

30cm knee wall at 25 degrees.

Then you should have balanced out the missing percentages.

But you do know the difference between [Kalt-] and [Warmdach], right?

But you do know what the difference between a cold roof and a warm roof is, don’t you?

Hm – you are probably right there. I have used the term more colloquially for “not intended as/for living space.” Should anyone have misunderstood this by accident, here is a quote from PREFA again:

A cold roof has, unlike the warm roof, a ventilation layer between the roof covering and the thermal insulation.
A warm roof is a roof construction where there is no ventilation between the roof substructure and the roof covering!
So the term "cold roof" should also apply to my case – the ceiling of the upper floor is insulated. There is still enough space with air circulation between the roof covering and the upper floor ceiling.

But I think that those reading along here have already understood that my roof is not suitable for living purposes and (I had written it under #31) the insulation of the upper floor ceiling was done.

However, I am not from the construction field and therefore tend to phrase things more in layman’s terms.

I think that a cold roof also won't fit for you - nowadays it is rarely built because it is more complicated.

My attic is completely unfinished (except for the 60mm wood fiberboards) and is still a warm roof.