Are there many plots similar to yours in your region? Then you might perhaps ask the advertising prefabricated house providers ... see which civil engineering companies work for the developers and maybe approach them directly. Perhaps they have recommendations/contact addresses for you or you can partially "tag along". Or is your plot unique in the region? What/how have the neighbors built? [...] I still can’t help further, but maybe others can.
The plot is, to be exact, unique in this forum, only and have somewhat comparable ones. I consider your suggestion to look around the area very helpful ;-)
Has the plot already been surveyed and could you possibly post the survey plan here anonymized?
That is already here, I linked it in post #2.
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We cannot say much about the old project, as it is under legal review.
Hints (or confirmations or refutations of my assumption) would already be helpful, insofar as I assume reasons that would also apply with a new construction partner. My assumptions point to the provider getting cold feet and/or having made additions to the offer that made the mutual expectations irreconcilable. Outlining this at least roughly, I consider useful for the discussion.
@11ant You said it would be madness to carry out the construction project with a general contractor (GC).
No, I said it would be madness to fixate on a GC from the outset (and it would be even more madness to even "plan" with a GC). A GC can be a good contractor after a tender, but instead of a tender, one should never take one (which applies even more extremely in the case of such special plots – so much so that even a Prussian would use the Bavarian "niemals nie nicht" (never never not) for it, and a genuine Bavarian would even say "niemals nie nicht, Deifi nochamoi"). This plot is quite literally an absolute contraindication for a plan donkey!
The plot also no longer looks like it did on the original plan. By now, you can even drive up there with a car.
A car maybe weighs two tons once, and is quite tolerant of being parked on a slope. Crane platforms are a whole different world.
He is currently just calculating with a volume. I don’t think much detail has been calculated yet (also difficult without finished plans).
It just seemed quite high for the house to us, and the GC’s statement in the last conversation was somewhat vague. [...] Is there any reliable source where you can get approximate values for construction costs? We would already be satisfied with just the pure house price, since we already have some information and comparison values for the individual earthworks, and yes, these can definitely reach six figures here.
Your plot is already, due to its topography, in the building ground class "unpredictable effort". There are no simple volumes for anything, and also no single volume for anything. Either the statements are vague or the mentioned worst-case figures are dizzyingly high. You can find similarly demanding plots at and Anson Argyris had already capitulated here in front of the same plot which has meanwhile built on. And you will also scare off several GCs yet – one more reason to at least carry out the planning with a suitably experienced architect. In this special case, even a "simple" house will not be a "simple house," and a "pure house price" cannot exist in this respect, because such prices assume ground slab-suitable construction situations. "Vagueness" here is "precisely" the factor before all principal cost figures.