Realistic financial planning for KFW40 prefabricated house?

There will be no underfloor heating installed,

This reduces the resale value of the house

We have demolition old building with basement. That means the "miracle box" earth disposal is not required for us, as we can use the earth pit of the old basement.

You should tell more about the old basement, it might possibly be integrated into the construction project (?)

One often reads as a tip that you should go to prefabricated house manufacturers with a finished architectural plan to compare offers. I can only recommend that to a limited extent so far.

I can also only recommend that to a very limited extent. By the way, with an architectural plan you do not "compare," but rather you "tender." That in turn is not possible with prefabricated house manufacturers.

I would definitely make a plan so that you get a realistic cost calculation, for example regarding the windows (cost drivers!).

a) What kind of planning do you mean there? b) In what way do you see windows as a cost driver per se?

But some prefabricated house manufacturers also have relatively good special offer houses that can be used as a starting point.

For heaven’s sake, anything but that! - Special offer houses are aimed at people who are satisfied with color choices and moving around two non-load-bearing interior walls. All changes before the decimal point confirm the second gastronomic principle.

For cost reasons, we want a utility basement,

See above ;-)

Now the house building companies calculate directly with insulation of all walls and the floor slab. The insulation must then supposedly be of the same standard as the house, [...] A prefab house provider told us that you can also simply insulate the basement ceiling, i.e. the transition from basement to house. That means KFW 40 house + uninsulated basement.

... or KfW40 house + KfW55 basement outside the "thermal envelope." In both cases "utility basement new" or "utility basement old," I would insulate the basement ceiling. And as always, I suggest thinking a third time about the avant-garde KfW fat level.

We have a plot of land in sight, so now I am dealing with the costs.

It's best the other way around: how much capital is available? You were probably already at the bank and know that. Then the land costs including incidental purchase costs are deducted. Then house demolition and ancillary building costs of about 30-40,000. What remains then has to be enough for the house, carport, and paving. You apparently already have the garden. If you calculate €2,500/sqm and take your space program, you will know whether a basement fits into the budget at all. Then you have to see which house provider is suitable: one builds cheap houses, the other expensive ones. Since you also plan upgrades that enhance the house, you have to see in which category you find yourself. Electrical work usually needs to be upgraded everywhere, but some house builders include more floor windows than others. One already has a fancy wooden door, another a cheap plastic door. You have to look at the building service specifications.

also have relatively good promotional houses that can be used as a jumping-off point.

Warning: promotional houses are hardly modifiable. At least in the prefabricated house sector.

Warning: Auction houses are hardly modelable.

I just said that ten minutes ago ...

At least in the prefabricated house sector.

... and it is not limited to that.

That is what I just said ten minutes ago ...

I must have already been writing then. Such things happen in the forum, that you cross paths. No reason to repeat ;)

That reduces the resale value of the house

Not even a line drawn yet, he already wants to sell again. :D