Is a house for a total volume of 400k still possible at all?

When did you mention that the property has a slope? With a slope, it looks different, especially if it is that steep. Then there is the possibility of a residential basement floor. It is probably the cheapest. Please upload a site plan with contour lines and the [B‘Plan]. Often there are height restrictions. Why can't you come to terms with a residential basement floor? They are normal, bright, friendly, and dry rooms. I have one myself.

The plot has not yet been surveyed, which is why I do not have contour lines. Since the terrace exit should be on the ground floor, the bedrooms must be downstairs. We have not thought about anything above that yet. There is also no development plan, which is why we have to adhere to the existing properties.

Gable roof
Max. 2 full floors
Certain distances from the street.

Very important, neither of the two sellers told you this: the statistics. There is so much steel in the wall on the side of the slope and in the floor slab that the construction company asked if we were planning a bunker. The ceiling had to be made thicker, etc., it was a five-figure amount. It would have come up with the cheap potato cellar as well.

I would approach the solid builders within a 10 km radius around the church tower. They know the conditions and they don’t care whether there are living spaces in the basement or not. Timber frame builders tend to build towers.
Without exact specifications
Basement: utility room, laundry room, bedroom, small bathroom
Ground floor: living/dining/cooking/WC
Top floor: children’s room, small bathroom

or
Basement: living, dining, kitchen, WC
Ground floor: sleeping, children, bathroom
Top floor: utility room, study

so use all floors and reduce the footprint.

How I love our bedrooms in the (Hang-)basement. Always nice and cool in the summer. You can subtract the cost of an air conditioner from the basement costs.

Included are also 2 small windows on the side not facing the slope.

I must have missed that now a house plan is being made based on the seller's statements? And then with a simple basement that even has two windows facing the garden side? :eek:
There are still several planning steps missing. The seller can't do them for you.
What is described there is not what anyone wants or costs more money than is suggested here. I can already see: next week Friday the construction contract will be signed o_O

I apparently missed that a house plan is now being made based on sellers' talks? And then with a single basement, which even has two windows facing the garden side? :eek:

I read it as the windows would be more towards the street side, since the terrace has to be on the ground floor. Therefore, I would have relocated the bedrooms downstairs :-). A terrain section would be really interesting. But in my opinion, a cheap basement would definitely be a waste.