Single-family house or semi-detached house?

I just can't wrap my head around it,

... and it doesn't have to.

why a joint planner/general contractor should be the only sensible way and only in this way can such a harmonious picture emerge.

From my point of view, it is simply more efficient to build the semi-detached house as a whole. In the interior finishing, everyone can still go their own way.

Exactly. Because of the whole. The autonomy of the two halves is overestimated. Everyone has their own land register entry, yes – but so does a condominium. The shared side remains an "interface."

And when built one after the other, it's even easier.

I wasn't born in an office, but have worked on construction sites. Therefore, I see it exactly the other way around: active coordination is always easier than first letting facts be created, then measuring them, and only then knowing the basis for your own planning.

No one has to celebrate their own topping-out ceremony a year later than the neighbor just to have a different house width and different interior wall positions.

Architect A surely doesn't make himself available to Architect B as a planning contact person or give him plan files so that he can ensure the fitting accuracy of his work at the interface.

And as I said: if Architect A turns out to be such a towering blockhead that you can't come to an agreement with him, you still have the option to go to his colleague.

Nevertheless, to take someone else from the start out of principle, I would call that "a stubborn child." It has something of a defiant phase.

And company xy is good, but expensive.[...] Where can we save on my half? Cheaper doors, Polish windows, concrete garage, and so on. [...] Then your half is simply the Hartz 4 version of the neighbor's house. So what.

Company xy is expensive when they build fancy, but fancy and upright don't really go together anyway. If the neighbor had too much money, he would have liked dozens of other plots better. There’s a reason why he ended up in Paderborn and not on Broadway. It’s fine, or will be made to fit.

Third and final version, if xy and neighbor are miserable. Then there’s no duplex. You and I both keep our three meters, maybe stick the garages together, and each build ourselves a two two-story bowling alley. Watch out, then he’ll soften up!

Two two-story bowling alley - You made my day!

You are telling the OP nonsense... one can only hope that he still seeks advice from another source....

If an architect wants to play general contractor, he is usually more expensive than a large house provider. You don't have to try to dissuade the OP with alternative facts from looking for cheaper alternatives.

And that talk about "unity" and the "whole" being nonsense can already be seen from the fact that both semi-detached houses can achieve different energy standards. One side is built to the Energy Saving Ordinance and the other to KfW40+

Semi-detached house can also simply mean that two houses stand very close together... you need to free yourself from your thinking that a semi-detached house is a horizontal two-family house.

Thank you all. So in Paderborn, the plot is not: the neighbor will buy the plot just like me because it is quite central in Düsseldorf and comparatively inexpensive - in that district, another plot is currently being offered for €1000/sqm. The neighbor will simply build, but the seller of the plot is ultimately willing to accept less than I am: he reduces the price based on the living area: 150sqm vs. the 200sqm I want. 8x12 would probably be the first idea for his building area. Since I am allowed to build max. 10x14: would you distribute the 200sqm over two floors and use the building area fully or possibly 9x14, instead of expanding a 2nd floor with a smaller building area? Is that cheaper in terms of construction costs?

I don't understand your question. One lists a need for rooms that relates to the present and future, to the size of the family and their habits and hobbies, what one needs or will need. Then one creates a spatial program indicating where which room is best located. After that, construction takes place. You have already written something strange about this above. One builds individually for oneself and not what one could. Regards, Yvonne

If I could choose between a detached house and a semi-detached house, I would always choose the detached house, even if I had to make one or two compromises regarding the floor plan or the external appearance of the building. Your discussion about being tied to a common planner, coordinating different planners, coordinating the exterior design, the timeline, etc. only confirms this. And later I live next to a hobby handyman who attaches shelves to the shared wall every Sunday morning.

But isn’t he out of luck if I want to build a single-family house?

Yes. In the unplanned inner area (§34 Building Code) the setback from the boundary would be the rule. Whoever wants to build on the boundary needs at least a declaration from the neighbor in which he also commits to building on the boundary. This can (but does not have to) be secured by a building encumbrance.
§6 Building Regulations NRW, 1
...
b) opposite those where building is allowed without a setback according to planning law regulations, if it is ensured that the neighbor’s property will be built on without a setback.

The municipality must have been crazy to allow that without both properties belonging to the same owner.

What does the municipality have to do with that? A development plan was not drawn up. Therefore, every property owner has free design options within the framework of §34 Building Code.

The first one infringes on the freedom of the other

No, because the first also has to ask the neighbor whether building on the boundary is intended. Only if the first builds with a setback does he not need the neighbor’s consent. If you want to build a semi-detached house, the possibility of boundary construction should be secured before purchasing the property.