How is the maximum allowable living area calculated?

Thanks to everyone for the clarification. My architect was right and I just had no idea :) We started with larger balconies & living spaces that were gradually reduced shortly before the application, which made me a bit "curious." Now, however, I can say in good conscience that the building plan does not offer much more and we have exhausted the possibilities. We will now proceed to the detailed planning for the interior of the house. I will create a separate post for this to carry out more detailed considerations. Thanks to all the professionals here :) I have become a fan of this forum.

We started with larger balconies & living spaces which were gradually reduced just before the application, that made me a bit "curious". But now I can say with a clear conscience that the building plan does not allow much more and we have exploited the possibilities.

Be happier with the house: try to use less to the maximum. Living quality does not come from having squeezed the building authority for the maximum cubic meters of enclosed space.

In my opinion, everything is quite well thought out. What I would carefully consider is the air space and the open staircase. That might look pretty nice. But the connection to the upper floor is often not that good. I had it like that too and then changed it afterwards. One more room and finally peace in the living room. I don't like it that much when I want to watch TV and someone is constantly walking along the stairs.

Maybe that counts as living space and messes up the whole calculation?

Is that common practice? To specify air space and then still build it as living space?

Is that common practice? Indicating airspace and then still building it as living space?

No! That is not allowed. But that is not stated anywhere here either. Common practice and allowed is, of course, the reduction of living space on the upper floor, among other things using an airspace.

Unfortunately, we have fully utilized the living space in the attic. Could we replace the air space with something without increasing the living area, or are we also allowed to, for example, turn it into a storage room? If so, I would of course want to proceed accordingly.