Bought an old cellar. Does it only carry wooden studs or also solid construction?

You need the structural engineer anyway, so why speculate about it now?

A contractor told us that concrete supports more load and that’s probably why in the planning back then, the masonry could be swapped with the thinner concrete walls

I see it similarly – for the house at that time, it was probably an adequate substitute for the originally planned masonry basement. Whether it is the same for a larger (taller) house now, well, ask the structural engineer. I would cautiously assume that he would advise positioning the load-bearing walls of the ground floor effectively in alignment with the wall layouts of the basement.

Okay, well I don’t have a single structural engineer on hand since we didn’t want to award the trades separately but rather build with a general contractor. I’m afraid I will end up paying for structural engineering twice – first a structural engineer to calculate which houses can be placed + then with the contractor everything will be structurally planned again for the actual house construction.

I see it similarly – for the house back then, it was probably an adequate replacement for the initially planned masonry basement. Whether it is the same for a larger (taller) house, well, ask the structural engineer. As a precaution, I would assume that he would advise positioning the load-bearing walls of the ground floor effectively aligned with the wall layouts of the basement.

Yeah, the new house wouldn’t be bigger or taller, just a different construction method – solid instead of wood.

You said a solid construction builder was concerned that the basement would only support a single story, without a developed attic. And I say, if the static calculation at that time was only designed for a one-and-a-half-story prefab house of that era, you have to expect that even a prefab house of current design is already heavier and would have to be reduced. The times when timber frame panel houses were lightweight are history. An energy saving ordinance "prefab house" has about 29 cm wall thickness, not 16 as it was back then.

Hm okay, we hadn’t thought that far. I wasn’t even aware that timber studs have developed like that! Good point. That’s right, in the ground floor plan (see attachment) the walls back then are specified as 16cm. Oh dear.. Well, if it’s only enough for a ground floor, then we’ll find something suitable! Now I’m curious what the first prefab house suppliers will say about it. So far, only this solid builder has commented on it.