Full story definition earlier

Hello everyone. Since our search for plots of land is going quite poorly, we are slowly playing with the idea of purchasing an existing building. One question we are asking ourselves is the possibility of adding an additional floor to a bungalow. For example, if in a development area from the 60s and 70s a full floor is permitted and a flat roof is mandatory, what does it look like with the addition of another floor? The definition regarding the full floor at that time is not entirely clear to me. Back then, the point "floor area of the floor below" was not specified. Does this now mean that due to the then 2/3 rule and the flat roof requirement, an additional floor is not possible? Or am I misunderstanding it?

1-story with flat roof:
Theoretically, only a recessed floor as an extension would be possible, where the additional floor may then have less than 2/3 of the ground floor area and thus is not considered a full floor.
In practice, you still have to check what the development plan allows, e.g., maximum height could impose restrictions, setback areas... and then there is the structural engineering – it must be able to bear the load. And since a recessed floor is offset from the load-bearing exterior walls, I hardly see a cost/benefit calculation there.

Does this now mean that due to the then 2/3 regulation and the flat roof requirement, no extension is possible?

Yes.

Or am I misunderstanding that?

No.

In theory, only a recessed storey would be possible as an extension, where the additional storey must have less than 2/3 of the ground floor area and therefore is not considered a full storey.

No, because the current definition of a full storey did not apply then, and at that time the 2/3 had to be related to the floor area of the storey under consideration. Although adventurous constructions would be conceivable here, such as lowering the clear height of non-habitable rooms, such cramped circumventions of the regulations usually lead to unattractive and impractical buildings. It would be more sensible to take a look around the building area to see how the existing development has evolved over the last 50 years and to inquire with the authorities to what extent the urban planning objectives from back then are still valid today, keyword exemption according to §31 Building Code.