How to divide land for a planned new single-family house?

Dear community!

I am very impressed by all the expertise here. Now I need help regarding property division. We have confusing and partly contradictory information about what is possible, and the responsible building authority is dragging its feet. We are planning the construction of a detached single-family house in a village in Baden-Württemberg. For this, the parental property is to be divided. It is developed with a multi-family house and a solidly built shed with an integrated garage. Adjacent to this is a paddock, where our house is supposed to be (see site plan). Half of the shed shall serve as a replacement cellar for us as well as a shelter for bicycles, strollers, etc. The other half with the garage can only be accessed from the parental side and is supposed to remain there. So far the plan. The shed is built of aerated concrete, 12.5m wide, 11m long and 5.90m high, has a gable roof (covered with Eternit panels) and a roof overhang of 2.90m towards the paddock. This could be dismantled.

There are now several options:
1. Divide the shed in the middle. Problem: The wall inside is not continuous. For a division, apparently a firewall is necessary, what would that cost?
2. Zigzag division: We divide inside along the wall, the garage remains in front, the rest goes to us. Too complicated?
3. We get the entire shed, the parents get usage rights for the garage.
4. The entire shed remains with the parents, we get usage rights. How big would the distance between the new building and the shed have to be in this case? Is it allowed for the boundary to run directly along the shed?
5. Partial demolition and new roof, whereby a) only the garage remains, b) the entire parental half remains and they receive the missing outer wall. Problem: Eternit panels, high costs??

What do you think? What would make sense? Have we not considered an option? The shed is huge, we actually don’t need that much storage space. However, we shy away from the costs of a (partial) demolition and the disposal of the Eternit panels.

Many thanks!

I am deeply impressed by all the expertise here.

Then you should have already read that expertise can only bear fruit on a foundation. We know too little about your situation and can only speculate: if we are talking about §34, is the yellow area at the bottom the street, is the access to the property concerning the parental house from above, do you want to build at the bottom, should your house be accessed from the lower street, ... - for now I see more questions than facts that are staring at me from the picture. Spontaneously, I would tend to prefer the footprint of the shed as the building site and find a replacement for the garage. Why is a division of the property desired?

Shed/scales as common property with corresponding special ownership usage rights would be the simplest solution. Then a straight cut for the new property.

Thank you for the note! It must be accessed from the top of the plan. The bottom of the plan has a slope, and according to the city, the connections are more than 70 meters away. And a division is desired because there are three more siblings.

Sorry, I haven't grasped the situation yet either. Why such a limited plan section? That's an old plan, isn't it? The red residential building is your parents', right? Where is yours supposed to go? Is there a development plan?

Show the entire property with access and sketch (better: map) the construction project.

And a division is desired because there are three more siblings.

I do not see a reason for cadastral land division in this. I can imagine both a hereditary building right in the desired building area and a separate condominium unit on the otherwise unchanged property. And with siblings, even a property company. There is either a dispute about the inheritance or not - if so, neither a straight fence nor a zigzagging one helps.