Build a prefabricated house over an old existing basement?

Good evening,
we have purchased a plot of land in a residential area including a 50-year-old prefabricated house with an existing basement. Renovating the house is not worthwhile, but we would like to keep the basement if possible and then have the new prefabricated house (single-family home) built on top of it.

However, the floor plan of the existing basement is smaller than the floor plan of the prefab house we want:

Dimensions of the existing basement (floor plan see attached file):
10.82m x 8.58m (but with an offset, i.e. 4.5m of the width protrudes 1.20m at the front and thus the remaining approx. 6.3m of the width protrudes 1.20m at the back).
Across the entire width, the basement also has another load-bearing wall in the middle.

Dimensions of the prefab house:
12.35m x 9.10m
+ 4.61m x 1.20m (flat roof bay window)

A connection between the basement and the house is not necessarily required for us, as the basement has an external staircase and is actually only intended to be used as storage space.

Is it possible to completely thermally separate the basement and the prefabricated house and possibly place a slab on the basement, or would that cause structural problems?

What minimum costs should we expect assuming that the basement is still very load-bearing and the ground does not hold any surprises?

Is something like this even worthwhile or should one definitely have a new basement built?

Of course, further suggestions or advice are very welcome.

Regards,
Dirk

A fifty-year-old basement would by now definitely show signs if it had been built defectively. If there is no indication of that, I would consider it as "foundations". That would already tick off the following points:

- Foundation present
- Connections already introduced
- Storage space that can be saved upstairs.

The load-bearing wall of the basement supports its ceiling. Where it is located is not significant for the prefab house above. However, it is not advisable to add a second ceiling (in the form of a floor slab).

One has more freedom in the planning above if the location of the stairwell does not have to be incorporated into the floor plan. However, this would still need to be closed.

It is most favorable if the new house basically "fits" over the basement, i.e. follows its outlines (a few cm overhang are not problematic).

Prefab houses can now also be re-planned with centimeter accuracy. That should not be a big deal for the architects of the house manufacturer.

If you upload the basement plan here again as an image file (jpg is preferred here over pdf), you can also draw the outlines of the new house right away.

Please provide the model name of the house so that one can take a closer look and offer re-planning suggestions.

Such a basement is a foundation even if the dimensions are not quite perfect, which should not be discarded lightly.

I see twelve risers on the interior stairs and ten on the exterior stairs. If they are equally steep, that would suggest a basement ceiling slightly above ground level (?)

Such a basement is also a foundation, even if not quite dream dimensions, that should not be discarded lightly.

I see that very differently.
Building over an old basement with a prefab house with a completely different floor plan results in a Frankenstein house. I also doubt the technical / structural feasibility.
Either a new basement or no basement at all.

Building over an old basement with a prefab house that has a completely different floor plan results in a Frankenstein house.

I would see it that way with free overhangs as well. But when you look at both (old basement and new house plan), you can already see how to stack them and also weigh the purpose.

I deliberately say "not discard carelessly" - I do not say "want to keep it at all costs."

Thank you very much for the initial answers and assessments. I am also aware that the whole thing would not be entirely unproblematic. In addition, the current basement is not at ground level (there is a step at the front to the house entrance and 2 or 3 steps at the back to get to the garden) and moreover, the floor plan of the new prefabricated house would overlap with less than a third of the existing basement floor plan.

Attached is a picture with both floor plans overlaid.

and furthermore the floor plan of the new prefabricated house would run on less than a third of the existing basement floor plan.

Attached is a picture with both floor plans superimposed.

I see it as less dramatic there. As I said, just name the model of the desired house, then we can think about it in more detail.