Build a terraced end house with an additional unit (GÜ) on your own

He had already roughly explained it to me: the base slab is then made as a precast concrete element and then rests on the foundations.

Therefore, no expensive compactable material needs to be poured in between, but the excavation can be used. The fill then no longer supports the base slab but the foundations.

However, if I now know the exact execution, I will gladly explain it here.

Foundation at the cellar depth of the neighbor? What happens to the space between the foundation and the top edge of the finished floor?

Sounds like a half-high prefabricated concrete cellar without access/without windows and it will also be excavated or disposed of!

To me, that doesn't sound like 15k€, but I'm happy to be surprised

A wall foundation at a depth of about 250, along the entire length of the house?

I already see the basement calling.

He had roughly explained it to me before: the base slab is then executed as a precast concrete part and rests on the foundations.
Therefore, no expensive compactable material needs to be poured in between; instead, the excavation can be used. The fill no longer supports the base slab but rather the foundations.

Sounds like a half-high precast concrete basement