Ok, the issue is now clearer to me. The moat running through the middle of the property unfortunately causes not only water in the soil but also different soil conditions in the area of the new building.
I can only strongly advise having a new expert report done. I wouldn’t necessarily choose the cheapest one either. I’m thinking about how I would approach the matter...
Regarding the costs: Given the circumstances, I think a basement will easily reach around €100,000. -Excavation -Dewatering -Possibly shoring -Foundation as a "gray tank"
A foundation with well rings or piles is definitely cheaper. I also throw the term vibro-stone columns into the mix (I’m a fan of those). The most expensive item among the options is definitely the site setup. Unfortunately, I cannot give you exact prices as too much has changed recently and it depends on too many factors. But the mentioned €30,000 for piles is realistic. Well rings are generally somewhat cheaper, as are vibro-stone columns. I still see the possibility of a shallow foundation as well. Plus a hillside drainage system that "dries out" the moat or channels it around the house.
Have another expert report done. Ideally with possible foundation heights (top edge of the finished floor), so the building waterproofing can be determined precisely. That’s where the next thousands go down the drain.