Single-family house or semi-detached house?

Construction contractor-land seller)
Building desire must be blind, your plot of land is becoming "more and more likeable" to me

Developer)
It doesn’t smell like that to me here – as I understand it, the plot can also be bought without a construction contract (?)

Construction contractor)
For the planning of the shell construction and the building envelope, I would use the same contractor. As the name part "-half" already says, the two buildings only form a whole together. Mixing may be possible and allowed, but I certainly do not recommend it. The semi-detached house is often sweet-talked as a single-family house that saves on lot width, but basically you are dealing with a vertical two-family house here. I would therefore discard the idea of another contractor for the "whole" half and address the contractor about a shell house that can then be outfitted differently. Of course, other contractors also offer houses even if they have a party wall next to the neighbor without distance. But let me present an argument for a common contractor in your mental cinema (mind you, for one house made of two halves): Imagine how smooth your gait would be if each leg had its own brain.

Garage)
Construction contractors typically do not hold shares in prefabricated garage manufacturers. The value creation at the garage comes from the wages of their masons. That is where their interest lies. Also, a masonry garage can be partially built as owner’s labor – that would then be your interest. However, you have another interest: a precast concrete garage (as a finished space unit) would offer more interior width due to thinner walls, which brings joy with every entry, exit, and door opening.

Front door five thousand euros)
Yes, that is ten thousand marks. And yes, in my school days you could still get a small car for that. But today that (including installation) is a "normal" price for an aluminum front door, if you don’t want burglars to laugh themselves to death. Of course there are lockable wall hole covers for a quarter of that, but that is a different category.

I don't think you have to or even should use the same construction company. Because a semi-detached house is by no means half of a house.

Rather, a semi-detached house is a house that is built on one side next to another house.

And for reasons of soundproofing alone, a separate foundation slab and a double party wall are very sensible.

Both semi-detached houses do not even have to be made from the same material. You can even mix timber frame and solid construction methods freely.

Rather, a semi-detached house is a house that is built on one side connected to another house. [...]

Both semi-detached houses do not even have to be made from the same material. You can even mix timber frame and solid construction methods freely.

Aesthetically uncoordinated, half an apple and half a pear glued together, it is a matter of taste whether one finds the result "interesting" or "punky". After all, when you come home, you never have only your house in view, but inevitably also a corner of the neighbor’s house. Raspberry ice cream with mustard may be possible, but it is not to everyone's liking.

And for reasons of sound insulation alone, a separate base plate and a double party wall are very sensible.

I took the liberty of assuming that as self-evident.

Whether the two semi-detached houses are mirrored or individual has nothing to do with the question of whether both semi-detached houses are to be built by the same construction company ...

And to continue the metaphor:
Not everything that limps is a comparison.

Whether the two semi-detached houses are mirrored or designed individually has nothing to do with the question of whether both semi-detached houses should be built by the same construction company ...

The same contractor does not necessarily have to mean mirror twins. But the shared side (with or without offset) and the façade design are easier to coordinate from a single source in such a way that a harmonious ensemble is created.

Of course, one can play the stubborn child in principle and have their own architect take the measurements at the finished adaptation object before planning. About as sensible as sending a text message to the classmate sitting next to you on the school bus. I still come from the generation that talks to each other and think quite simply here:

If the neighbor’s architect as the land seller was not a big disaster, he might also be pleasant enough as a person to collaborate with to have a joint planner with the neighbor. In the TE’s place, I would at least consider this favorably.

You can still go headfirst through the wall afterwards.

I read, a semi-detached house is clearly more reasonable. Unfortunately, the partner is already firmly tied to company xy. And company xy is good, but expensive. Name interests. Discuss the case with company xy. Clearly say, let’s find a way so that I don’t have to pay real estate transfer tax on the entire amount. Important for me. Clearly say, I cannot and will not afford the 500 thousand. Where can we save on my half? Cheaper doors, Polish windows, concrete garage and so on. Pull rabbits out of the hat and see what’s possible. Then your half is simply the Hartz 4 version of the neighboring house. So what. If you can’t manage, then honestly say, dear neighbor, it’s all well and good that you want to go with xy, but I cannot and do not want that. Are you really that fixed on them or can we find a cheaper one? Third and last version, if xy and neighbor are miserable. Then there will be no semi-detached house. You and I, we both keep our three meters distance, maybe clap the garages together and each build ourselves a two-story bowling alley. Watch out, then he’ll soften up! Karsten