Hello MelW2024,
we have the same situation here. Old development plan from the 70s and only a knee wall of a maximum of 50cm. Gable roof.
I already wanted to write off the plot because of that, but the development plan also states that two full floors are allowed. From a certain knee wall height, a floor is considered a full floor; we are going for a knee wall of 2 meters which we wanted anyway), which solves the problem. We asked the responsible building authority for the current definition of a full floor valid for the building area. Of course, that depends on the federal state.
Inquiries with the municipality and the responsible district office were very positive for us; both clerks had full understanding ("nobody builds like that nowadays anyway") and already indicated that a change request with the building application would not be a problem (that was our plan B).
How is it with you, how many full floors are allowed?