Floor plan and space issues for a single-family house

So the house should rather be directly on the boundary and not like the neighbor.

I don't know. According to current law, the building would not be permissible there. If the minimum distance is undershot, a building encumbrance would be required. However, this cannot be registered since there is already a house there. I cannot assess whether the conditions for approving a deviation are met.

The friendly civil engineer was first asked to handle the part of the preliminary building inquiry,

A preliminary building inquiry is no child's play either. You really have to consider carefully what you want to have clarified in advance.

It's first about determining a suitable floor plan (room layout and size, etc.).

Here the cat bites its own tail. What good is discussing the dreadful pantry if the building itself is not permissible at all? Without a positive decision on the preliminary building inquiry, there is no permissible location for the building and thus no floor plan. Without a floor plan, no building, and without a building, no preliminary inquiry. I recommend first conducting a surrounding area analysis to work out the integration criteria required under §34 of the Building Code (type and extent of building use, construction method, buildable plot area), then verbally exploring the possibilities and constraints with the approval authority, then designing the floor plan and submitting the planning law preliminary building inquiry. After a positive decision, proceed with fine-tuning, including discussion here in the forum.

then orally explore the possibilities and constraints with the approval authority

That was the very first thing I tried as well, but I was just brushed off with the explanation that each individual case needs to be considered separately and that I should submit a preliminary building application. They were unable or unwilling to provide me with any information, which is why I proceeded with the preliminary building application.

Who should I best contact regarding building on existing structures, etc.? The civil engineer was basically contacted because he is authorized to submit building documents and does not charge any money. He himself also said that he has little experience with the laws, etc., as he works as a construction manager in his company and not in the design/application area.

Thank you very much for your detailed help.

One could or would not give me any information, which is why the step to the preliminary building inquiry followed.

Yes, it is very unfortunate that public service employees often forget that they serve the public, i.e. the citizens.

and no money is charged.

That is of course an argument. Maybe the preliminary building inquiry with this floor plan will actually help you. I just fear that adhering to the existing structure has forced you and/or the planner into a corset that does not exist at all. An experienced planner with knowledge of local customs will, on the other hand, formulate the right questions so that your dream house becomes approvable.

I would wait with the demolition. I don't know if it's like this everywhere, but here you can basically build a new house within the old walls. Then you have protection under the conservation law, which expires if the old structure is first demolished and then newly built. That's why many people here build inside the old house. These are all new walls, but a proper "demolition" does not take place. It's more like an exchange stone by stone. This way you can preserve the old shell (and do not have to comply with the currently valid setback distances), but have basically built a new house. This is a little loophole here. So it's not a new building, but a conversion/renovation/refurbishment of an old building. During the conversion/renovation/refurbishment, it then becomes clear that all the walls need to be renewed...

I don't know if this works the same for you, but then you could keep the current boundaries. Escroda is definitely the expert on this.

That is why the old cellars often remain here.

That would be Plan B in case the tree preliminary request is rejected.

I will draw a more conventional floor plan over the weekend.
The difficulty lies in the fact that no windows are possible on the east side, the plot is very narrow, there should be enough storage space, and I would later like to divide the house into 2 units.

If I can't manage anything with that, I will most likely cancel the 2 units and plan with a normal single-family house.

Thank you both very much for the information.