For existing buildings, that doesn’t matter, does it?
That depends. In the case of a complete core renovation, possibly involving a change of the building class or the creation of new living spaces, the building regulations must be complied with for the new living spaces.
In the case known to me, it concerns the barn of a manor with partially installed living spaces after 1945. The new rooms (conversion of the barn and stable) must in any case comply with the state building code. The apartments in the area of the former residential and utility rooms also basically have to comply – but here there are restrictions imposed by the monument protection office, so two authorities have to agree whether the state building code may be violated or the monument protection requirements apply (usually there is then an exemption from the state building code).
Since this is not a listed building, the question is what happened during the renovation. If new living space was created (e.g., by converting old utility rooms), these must comply with the state building code. If rooms before the renovation had 2.3 m (ceiling height) but later no longer do, I could also imagine this being critical.
But (!), this is mainly interesting for rental purposes, whether I can officially count the m² as living space accordingly. In a house owner-occupied yourself, it only bothers yourself in case of doubt. Well, when buying or reselling, the living space must be stated correctly and this could possibly have an influence on the nominal "value" of the house. However, this should not be confused with the value it has for the resident/buyer.