Properly dividing land for single-family house + duplex

Thank you both for the numerous hints. This has already given me quite a bit of material for my own thoughts and planning. Now, an interesting mix of planning law, tax issues, and contract topics seems to be coming my way.

Now an interesting mix of planning law, tax issues, and contract topics is apparently coming my way

Would come, subjunctive. That would be the case if you were in the situation of having the money ready for the development of the entire property. As I said, I see two other things in front of you first:
1. Formulate what YOU want to realize on this property;
2. Make sure that the property owner wants to go this path with you.

Planning law)
Take your single-family house. Whether the property owner then subdivides the rest again or builds the semi-detached house as a WEG (condominium ownership), that can be irrelevant to you. You will be sufficiently occupied with your thoughts.

Tax issues)
All three or four home builders buy directly from the owner of the entire property; that is not complicated.

Contract issues)
Beware of the crazy idea of forming a demolition project company with the other home builders. Buy "already demolished."

11ant: Thank you for the advice, I will take it to heart. Following all tips at once will be difficult to achieve in the current seller's market, but I will try to limit my risk... Or give up on it in the end.

All advice at the same time will be difficult to fulfill in the current seller's market,

At least my advice to you is not overly numerous. Essentially, I have given you one piece of advice in two parts:
a. get clear about what you want
b. infect the current property owner with this idea.