Building in existing structures - Subsequent additional costs despite a fixed price

He should first prove that he does not owe the additional foundations.

I am still missing the plans of the new house to compare them with the old basement. But tendentially, I still firmly believe that the supposedly necessary reinforcement of the foundation will be magically done in a split second if the costs remain partly with the contractor.

Alternatively, you have the legal route via lawyer / legal advice.

I interpret the lawyer's dispute-related business fees here as an ulterior motive behind the 50k heavy threat scenario - the contractor wants to convince the client that the learning fee is a smart choice ;-)

The floor plan of the basement itself remains unchanged, as a new ground floor and a new upper floor are being built on the existing basement, with an identical footprint to the original. The deviations in the floor plans of these floors certainly have little impact on the load transfer.
The construction of the walls definitely has nothing to do with how prefabricated houses are built nowadays. According to the historical execution documents, the load-bearing exterior walls have a thickness of 14.6mm with the following structure:
12.5 mm plasterboard
10.0 mm chipboard
0.1 mm polyethylene film
90.0 mm timber framing
80.0 mm thermal insulation
10.0 mm chipboard

Moreover, it should not be overlooked that the new intermediate ceiling is made of precast concrete elements. I generally consider it plausible that the old foundation cannot bear the load of the newly planned house, but of course I am not a structural engineer myself and have never seen the new structural calculation.
Am I basically entitled to these documents?

What do you mean by your second paragraph and the 50k heavy threatening scenario?
That the contractor knows that with so much money invested, 50k already paid + 17.5k additional claim, the lawyer fees are more deterring than his claim for the foundation of 17.5k and he therefore thinks we will agree to cover the costs?
Unfortunately, I cannot fully grasp the point you want to make here, but I would like to :)

The floor plan of the basement itself remains unchanged, since a new ground floor and a new upper floor are being built on the existing basement, with an identical footprint to the existing structure. The floor plan deviations of these floors certainly have little influence on the load transfer.

Uh, no, exactly the opposite.

Moreover, it should certainly not be neglected that the new intermediate ceiling is made of precast concrete parts.

Oh, that too, the basement ceiling is being renewed?

What do you mean with your second paragraph and the threatening scenario weighing 50k?

I understood it as meaning that in a contentious dispute, lawyer fees and court cost advances are based on a dispute value of 50,000 EUR. Court of first instance dimensions are no small matter and certainly serve as an intimidating prospect, effective as a quasi-argument for a "small concession."

I would like to see the calculation of the [Zusatzvereinbarung].