Basement walls look bad!

Aha, very interesting. Learned something new again, thanks. Uff, wouldn't know if I would recognize something like that.

What is at the border with the neighbor? Just pavement or something similar? I say 3500€ and a week of hard work and the deal is done. Mini excavator costs 120€ per day, dig for one day, then let it dry. Then if it's already open, definitely insulate the wall! Material is cheap. Then put in drainage and cover it up. Make a deal with the neighbor, even if he would have to tolerate the work anyway. If water is pressing from below, however, that's a different story.

But it is strange that the ceiling height in the basement is only 1.85m... while the old plans show 2.20m.

That is indeed the case. A 35 cm difference between the plan and the actual measurement seemed worth clarifying to me as well. They certainly didn’t remove that space when cutting through; they only added insulation. However, a 185 cm high basement in 1955 wouldn’t have been unusual: it was a frugal time, basements were used to store potatoes, which don’t need much headroom, and people didn’t spend much time down there. At 185 cm, you can still walk relatively comfortably bent over and only have to duck your head occasionally under pipes. Economically, it was a good compromise between crawl space and regular basement, from the perspective of that time.

First of all, thank you very much for your answers! At the border to the neighbor there is a paved driveway with an embedded flower bed which directly adjoins the house. We have looked again and the walls felt relatively dry. However, it is possible that the 10+ pieces of Uhu dehumidifiers lying in the basement have contributed to it now seeming drier. The basement is also unheated and has only a very small window. Would it help to have radiators installed?

How about a moisture meter? And before buying the thing, you put down 2 hunnis and have an expert on hand who inspects the house.

You can certainly get a moisture meter. Of course, an expert would be the best choice, but my son thinks that's nonsense. "It's just an old house, and the basement is allowed to be not completely dry sometimes," he says.