ᐅ Waterproofed Basement – What Additional Costs Can Be Expected?

Dear experts,
we (4 parties) are currently building a 4-family house. The basement and the walls on the ground floor and first floor were supposed to be completed by now. Unfortunately, groundwater was discovered during the excavation of the building pit. This has, of course, changed a lot.
The basement builder now demands approximately 36,000 Euros in additional costs for a waterproof (WU) basement compared to the originally planned basement. Are these additional costs realistic and normal?

Originally planned:
Reinforced concrete slab 18 cm (7 inches) thick on a foil covering (approx. 150 sqm (1,615 sq ft)),
Gravel filter layer 15 cm (6 inches) thick with area drainage function,
Shotcrete walls C25/30 20 cm (8 inches) thick with bituminous thick coating on the outside and perimeter insulation (approx. 125 sqm (1,345 sq ft)),
Clear height of basement rooms 2.50 m (8 ft 2 in);
Originally planned construction costs for the basement: approx. 100,000 Euros.

Now planned for the waterproof (WU) basement:
Reinforced concrete slab 30 cm (12 inches) thick,
Increased consumption of reinforcing steel,
Gravel backfill 1.70 m (5 ft 7 in) around the basement as a drainage layer,
Concrete quality C30/37,
Increased excavation volume,
Clear height of basement rooms 2.30 m (7 ft 7 in);
Additional costs for this: 26,900 Euros.
In addition, waterproof construction charges for concrete additives, special construction materials, pipe penetration elements, and construction services amount to another 6,500 Euros.

This results in a total of 33,400 Euros in additional costs.
Can you assess whether these additional costs are reasonable or if these prices seem exaggerated based on your experience?

I would be very grateful for your evaluation.

Best regards
Steffen Frey

The new car from the boss is already included in the price. You won’t be able to change that anyway.

Lumpi_LE schrieb:
The boss’s new car is included in the price. You can't change that anyway.

I wouldn’t generalize like that. There are clearly additional efforts involved. Just the foundation slab is almost twice as thick. Whether the price is justified, I have no idea. Maybe you negotiated poorly beforehand. I think the price increase is reasonable proportionally when scaled down to a standard single-family house basement size.

Lumpi_LE schrieb:
The new company car is included in the price. You won’t be able to change anything about that anyway.

Keep in mind, this concerns a four-unit building, not a single-family house.

I am a bit confused. We have the same values for the waterproof concrete structure. The only difference is that our slab is 25 cm (10 inches) thick (I am referring to the baseline). I would suggest having this checked by an external expert.

I consider the additional cost compared to the basement price of 100,000 euros to be quite realistic. A thicker foundation slab, more steel reinforcement, and so on—that simply costs more. A friend of ours also had to build a waterproof concrete shell (a "white tank") back in 2006 and already faced an additional cost of 30,000 euros at that time.

Best regards,
Sabine