Property purchase & broker behavior: Is this normal?

So it’s from the realtor now?
Spontaneously, at the beginning of your thread, I thought this was again one of those topics that the forum search term "11ant Leerverkauf" would lead you to – but at least, that doesn’t seem to be the case here. Nevertheless, I dislike the situation here that, if I understood correctly, you are supposed to buy the plot separately despite the compulsory developer clause. That smells a bit like the fairy tale that you can avoid paying property transfer tax on the house that way (which the tax office sees differently; there are plenty of threads on this here as well). By the way, it wouldn’t change anything if the parents bought the plot.
Two other things don’t quite fit for me here: on the one hand (given the current market situation, incredibly generous) almost a thousand square meters, which is still quite a lot even with a floor area ratio of 0.2, but on the other hand the remark that the plot is suitable for a semi-detached house. Well, if it can be developed as E/D [Einfamilienhaus/Doppelhaus - single-family house/semi-detached house] – but nobody puts a semi-detached house into such a park. The other thing is that your self-diagnosed not-yet-existent building readiness and this size of land do not match. Whoever doesn’t yet know what they want but is willing to accept a semi-detached house is probably interested in such a large plot only together with the buyers for the other half of the semi-detached house. Alone – meaning as a single couple/family – you rather look for a plot of this size with already very mature, concrete ideas about the house, and when the sum of children and cars is about seven.

How is it of no use to buy that separately?

What I don’t understand: Why shouldn’t you build a semi-detached house in this "park"?
So we already know what we want: We want to build a semi-detached house, the other half will be occupied/rented by my parents.

Sure, we could have the land divided and buy their share from them. But what for the additional costs? If we’re going to live there together anyway.

I am not accusing you of intending to evade taxes, but it is known that property developers like to provide misinformation in this regard. Because the tax office will always consider a developer's house and its land as a connected transaction.

That would of course be annoying, since there is also a house on the property that we would have to demolish.

At first Google search, I found the information that this can only happen if the purchase contract states that we are bound to someone.

My initial question was actually quite different and in my opinion these details were not yet relevant because I first wanted to know what real estate agent experiences you have.
For that, it was still unimportant whether the property is developed or not.