Estimation of semi-detached house construction costs

we also calculated with 2000/m2. But the other 2 architects said 3000€ and up. We already found that quite a lot, even for BW. The other things are clear!

on the ground floor still an office or bedroom for old age,

I would avoid that at your age. In your generation it will increasingly be the case that people build a second time later. You should consider this before contorting yourself in the floor plan design for it.

that doesn’t exist yet. The plot has not been sold yet. But the purchase contract only states that we must have the same roof pitch and tile color. The rest doesn’t matter at all.

In my opinion, semi-detached houses—as the name suggests—are not really suitable for uncoordinated building. Without a basement, you have a good chance that the other twin thinks the same way. But there are still plenty of problems lurking: A wants a gable roof, B wants a recessed floor; A builds “solid,” B builds “prefabricated”; A is finished and B hasn’t started yet: how do you insulate and protect the wall on the “shared” side? — and the like... The more “doesn’t matter at all” applies to you too, the better: choosing the same architect as the twin (which by no means has to mean “identical” houses!) simplifies the project in my opinion considerably.

we also calculated with 2000/m2. But the other 2 architects said 3000€ upwards. We found that quite a lot, even for BW. The rest of the things are clear!

We are building about 50km away from you, also in the "Bauhaus style". We are well below 3,000€, probably more like 2,300€/sqm...

The €3000 estimate can fully or partially include the sampling reserve, ancillary construction costs, necessary new installations, outdoor facilities to create a realistic sense of the construction? I mean, a 30% surcharge on the actual house price would be realistic for that at first.

Yes, as a complete package without the plot, the 3000 times the living area would be a good benchmark. At least it fits for us.

I totally agree with you! Unfortunately, the other plot has not been sold yet. We would have no problem building with the same architect if everything fits. According to the development plan, only gable roofs are allowed! We are aware that we have to come to an agreement with the "other side." We are quite open about that. Let's hope the "other side" feels the same way. Well, I think with a semi-detached house you have to make compromises, otherwise you shouldn't buy such a plot. We'll see...