Plot ratio after actual division of the property for a semi-detached house

No, regulate the entire property only nominally with special usage rights. Also the path.

Wouldn't you have problems if one of us sold one half because of this? I mean, the bank wouldn't agree to that either when it comes to financing!?

One more note:

On the left property - approx. 246 m2, the calculation is 246*0.4=98.4 m2. If I understand correctly, this fulfills the floor area ratio 1, since the house alone is approx. 97 m2. The garage is not taken into account for floor area ratio 1, right? Or am I wrong?

Here is my calculation (see image):

Wouldn't there be any problems if one of us were to sell one half?

A purely ideal division is not financeable for a bank, and the half is also not fungible. What is meant is a division into condominium ownership. That means two apartments with exclusive usage rights to gardens, paths, etc.

Wouldn't there be any problems if one of us were to sell one half? I mean, even with the financing the bank wouldn't go along with that!?

Just as @nordaney writes. It is completely common. With every condominium and many semi-detached houses, it is handled that way. Pushing the floor area ratio to the limit like that is foolish. Possibly the roof overhang is also counted towards the floor area ratio.. oh no, not in BW, but who knows how the building authority or the architect calculates. Because often the way a hand sketch is drawn is not how it is sensibly built. And your sketch raises the question whether the house is actually built as drawn. So if you calculate the access road, you are not doing yourself any favors.

My brother and I recently purchased a plot of land of approximately 596 m2. We are from Blaubeuren (Baden-Württemberg).
Our wish is to build a semi-detached house. Both halves should be as equal in size as possible. Since the plot is "trapezoidal" and we based the blue building boundary as a basis (for a real division), one half is smaller – about 246 m2.

I’ll reply over there where the expert is also available, and your brother "mechul" asked the same question. As it has been presented for discussion, in my opinion this is a "Pl." (= planning without any understanding).