What counts as living space?

hello,

I have problems after purchasing a new building with the calculation of the living areas. The contract does not specify any measurements according to DIN or any other standard, there is only a copy of the plan with all the individual values included 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 taken? Are there lists on how to measure what?

thank you for your answers

Living area

I think the living area includes everything that is walkable. So not the exterior wall, but the interior wall...!

Hello Bäumle, Nowadays a room is already measured with a leveling instrument. When measured this way, the entire floor area counts. For the walls, of course, the windows are deducted. Have a technician come who measures everything exactly.