Planning a land purchase and a 400k budget for house construction

If we go with basement/cellar and ground floor, we would have to completely redesign everything and possibly also switch to solid construction, since basement + prefab bungalow would be more expensive according to my research.

You mean: hybrid construction with basement in stone and ground floor in wood would be more expensive than both in stone?
By the way, I don’t see a partial basement here because the utility connections will probably come from under the street – that is, the uphill side – to the house. A partial basement would only make sense on the downhill side; otherwise, the house inlets would be built over (unless they are located on the side). So I assume that the most economical option will probably be a full basement (with the downhill side as a living basement). And: if hybrid construction is used, then with the ground floor builder as the general contractor for the entire house.

You mean: a mixed construction method with the basement in stone and the ground floor in wood would be more expensive than both in stone?

That's how I meant it, but as you said, the most sensible thing will be to have the entire construction project done by one company. That will be the easiest way. The utility connection is definitely from the road above. The sewer invert there is approximately at 416m.

That's what I meant, but as you said, the most sensible thing will be to do the entire construction project with one company.

As the person responsible for matching the lower and upper parts, yes. But not necessarily in a homogeneous construction method. I have just questioned the idea that mixed construction methods are generally more expensive (because in my opinion it depends on the individual case, insofar as a price difference is more related to the standard than to the construction method, i.e. normally the same standard is equally priced regardless of the construction method).