What counts as living space?

hello,

i have problems with the calculation of living space after purchasing a new building. the contract does not specify dimensions according to din or any other standard, only a copy of the plan with all the individual values can be found in the purchase contract. these specifications are apparently too low. which standard is generally common here? how is it for example with doors, etc.? is the floor area fully included? where can one view these various din standards and how the measurements were made? are there lists on how to measure what?

thank you for your answers

Here is my opinion:
So the Wohnflächenverordnung Wohnflächenverordnung and the German Industry Standard DIN-277. Publicly subsidized apartments or also owner-occupied homes must be calculated according to the Wohnflächenverordnung Wohnflächenverordnung!
But in privately financed residential construction, it is optionally possible to calculate according to the Wohnflächenverordnung Wohnflächenverordnung or also according to the German Industry Standard DIN-277.
In the German Industry Standard DIN-277, however, no living areas are determined anymore, but only the usable and traffic areas...

And how is it with the balconies? here it says 1 / 4, but it is supposed to be able to go up to 1 / 2, according to the law?! for me, 1 / 2 was credited. for my neighbor who lives on the same floor, 1 / 4? that is probably not normal!! the equipment is the same, floor area around 7.5 sqm, no canopy, can be seen into by various neighbors... and from 3 sides. but at least it has an outlet and electric light. but I think that does not make it particularly equipped either. it will probably be a lawyer thing again. since it is about a few thousand euros refund in my favor here...!