Expanding OG room - Is approval necessary?

Currently, the room is not included in the living area calculation and is not indicated as "Speicher" in the floor plan.

So what? What's the matter? Don't worry about it.

Nice of you to ask. Don't ask any further and just do it. Of course, you can also make a storage space a bit cozier.

Building permit with structural analysis, energy certificate, and so on. In the end, it turns out that the ceiling layer is 0.5 cm too weak and residential use is not approvable. A lot of money for an uncertain outcome.

Underfloor heating, screed, electrical installation, drywall, plastering, painting

All of that is not prohibited in the "attic".

Building permit with structural analysis, energy certificate and so on. In the end, it turns out that the ceiling layer is 0.5cm too weak and no residential use is permitted. A lot of money for an uncertain outcome.

all that is not forbidden in the "attic".

Thank you for your answer. How about an additional shower + WC + washbasin?

If I understand you correctly, I need a structural analysis (from a structural engineer or architect?) and an energy certificate (!= energy performance certificate?) – from where and for what purpose?

Which requirements for a building permit for which purpose can be set, I unfortunately cannot tell you either. I just wanted to point out that a building permit can sometimes be no joke and not proportional to the benefit.