Sequence in house construction in difficult starting position

helpful criticism would have been better

I really put a lot of effort into it, but after reading the original post I cannot understand the reality. Presumably many others feel the same way, which is why they cannot really answer your questions.

After rereading, the procedure is actually quite simple, isn’t it?
1. Demolish the building (garage and shed remain standing)
2. Divide / sell the property
3. Start building

The more difficult question is, who covers all the costs if one party is still undecided? There are three parties:
- Current owner (family)
- The thread creator
- The undecided third party
If the first two agree, can everything simply start without party 3? Can you morally or financially afford to simply bypass party 3 and present them with a fait accompli?

The whole thing is not that simple and therefore probably difficult to implement in the near future.
Regarding the attempts at explanation. The plot 34 and 34a is built in such a way that construction must be attached at the property boundary to plot 36. (Adjusted according to the development plan and to be secured with a building encumbrance)

Plot 36 is family-owned and currently partially inhabited with a rented old building. From 01/22 one or two parties can build here.

The following basic options exist:
A: One party (me) builds, then this must be built in the right, yellow area and the left, red area should, if statically possible, not be demolished.
B: Both parties build in the near future. The existing building will be completely demolished. This would be best but there is no agreement on when the other party exactly wants to build.

Additional options:
C: Each party builds a detached house, which requires no compromise. The middle on the property boundary would thus be lost building area.
D: A joint development in the middle, access and a joint building in the middle is carried out. High compromise necessary

The whole thing is not that simple and therefore probably difficult to implement in the near future.

No, I find it simple: Without demolishing the house, you won't get parceling, and without parceling, you definitely shouldn't break ground. So demolish, divide the property, and then build.

Until you submit the application for parceling, consider whether the new houses will be built attached or if you will leave space between them. I strongly recommend option C: Each on their own.