WU-Keller - What additional costs are to be expected?

Dear experts,
we (4 parties) are currently building a 4-family house. Actually, the basement and the walls on the ground floor and upper floor should already be standing. Unfortunately, groundwater was found during the excavation of the construction pit. Of course, this changed a lot.
The basement builder is now demanding approximately 36,000 euros in additional costs for a WU basement compared to the originally planned basement. Are these additional costs realistic and normal?
Originally planned were:
Reinforced concrete slab d=18 cm on foil cover (approx. 150 sqm),
Gravel filter layer d=15 cm with area drainage effect,
Shotcrete walls C25/30 d=20 cm with bituminous thick coating on the outside and perimeter insulation (approx. 125 sqm),
Clear height of the basement rooms 2.50 m;
Originally planned construction costs for the basement: approx. 100,000 euros.

Now to be installed for the WU basement:
Reinforced base plate d=30 cm,
Increased consumption of construction steel,
Gravel backfill 1.70 meters all around the basement as drainage layer,
Concrete quality C30/37,
Additional excavation of the pit,
Clear height of basement rooms 2.30 m;
Additional costs for this: 26,900 euros.
Additionally, waterproof construction charges a further 6,500 euros for concrete surcharge, special building materials, pipe penetration elements, construction services.

That makes a total of 33,400 euros additional costs.
Can you understand these additional costs or are these, in your experience, unrealistic prices?

I would be very grateful for your assessment.

Best regards
Steffen Frey

The new car from the boss is included in the price. You can't change anything about that anyway.

The boss's new car is included in the price. You can't change anything about that anyway.

I wouldn't generalize it like that. There are clearly additional efforts involved. Just the base plate is almost twice as thick. Whether the price is justified, no idea. Maybe you already negotiated "badly" beforehand. I think the surcharge is proportionate if you scale it down to the size of a standard single-family house basement.

I am a bit confused. We have the same values for [weißer Wanne]. Only that our slab is 25 cm thick (I am talking about the starting point). I would have this checked by external experts.

I find the surcharge compared to the basement price of 100,000 euros quite realistic. Thicker base plate, more steel, etc., that costs money. Our acquaintance also had to build a [weiße Wanne] (but that was in 2006) and already had a surcharge of 30,000 euros at that time.

Best regards
Sabine