How much additional cost according to the structural engineer's calculation?

There were almost 1000m3 of clay soil due to the slope and the backfilling with new material was also included. Clay soil is not suitable for backfilling....

By the way, I can't find this model in the HdH program at all, only as a semi-detached house (6.13 x 10.67 m) (?). What did you inquire about with them (as well as with the other two)?

As an assistance for the OP: For the re-planning from "without cellar" to "with cellar," the general contractor only charged 2500 euros. However, I doubt that it will be just as cheap the other way around...

By the way, I suspect that twenty thousand mentioned casually on the phone and double that on the invoice might be roughly adequate ;-)

But you can quickly calculate that.

For example: 11m*11m*3m = 363 m3 * 56 euros = approx. 20k