Which prefabricated house provider - prices, experiences?

We were actually thinking more of a terrace on the northeast corner,

Wouldn't northwest be better then ... evening sun and all ... and, terrace counts, as far as I know, as something that is not simply allowed to be built within the 3m boundary distance.

Unfortunately, the neighbor does not want to sell (we also once had the idea of simply buying and combining the rear parts of both parcels). But he also has grandchildren who want to build. Unfortunately not right now, otherwise a semi-detached house would have been possible. He does not want to give up that one meter either, because otherwise it will become definitively difficult to build on his property, which is already a bit narrower than "ours".

Since the neighbor will have a similar problem with building as you do now, how about a semi-detached house? If you both could imagine that, then why don’t you go together to the building authority and ask if that would be possible. In case of any objections, the number one argument would be the currently politically desired "infill development." The neighbor would probably have to divide his parcel already now, and you would have to temporarily insulate your house along the dividing line.

Regarding the land registry costs for the neighbor’s parcel division, I would, in your place, possibly even cover them; you would benefit enormously from this. The only bitter pill: no light on one side, but you would have a lot more of your property, especially an approximately 6 m wide west terrace plus the north garden. Or a 4.5 m west terrace and a house 8.5 m wide instead of 7 m, which would allow completely different floor plans.

To the northwest, we are completely built-up, but to the northeast it is still noticeably more open at least for now. You don't have to build in the border area either. For us, 4x3 is sufficient, that should be noted.

I consider the moisture issue in solid construction to be a myth or maybe it was like that in the past when real mortar was still used for masonry. The moisture comes into the house with the screed, and you always need that. Our new build had no increased moisture at all after moving in; we never went above 50% humidity. In our previous house (also newly built), we had a leaking water pipe in the wall after a few years (the pipe had slipped apart). As a result, water ran into the masonry all the way to the basement, and we only noticed it because of a wet spot on the outside wall. It was easy to repair, the bricks were dried, and then everything was fine again. I don’t want to think about what the walls of a prefabricated house would have looked like if they had been filled with water like that.
You should definitely calculate four meters of depth for the terrace; the table is one meter deep, and then you have 1.5 meters on each side for the chairs, so I would also try to place the house as close to the street as possible. Here with us, there are also many small plots with single-family houses built on them; some of them have only 1.5 meters between the street/sidewalk.
Best regards
Sabine

Definitely an exciting project.

It would be interesting to know what is allowed in terms of height. On the screen, the neighborhood looks like it has gable roofs. How many floors do the others have?

A semi-detached house would really be a great thing here, but I see no chance at all that the neighbor would be persuaded now. I would still try it though.

It would be interesting to know what is allowed at that height. In the screenshot, the neighborhood seems to have gable roofs. How many stories do the others have?

§34, 2 VG are allowed