ᐅ Slab on Ground in Contact with Water – Alternatives to Waterproof Concrete?

Hello and good day!
I’m new to the forum and have registered because I have a question about our building project:

We are constructing a single-family house in the South Heath area. Timber frame construction, one and a half stories, 170 m² (1,829 sq ft) of living space.
The soil report showed that the groundwater level in the built-up area is between 1.2 and 1.0 m (3.9 and 3.3 ft). A borehole next to the built-up area showed 0.9 m (3 ft). Above the topsoil is fluvial sand, gray → medium sand, fine sandy, weakly coarse sandy.

Originally, a slab-on-grade with a frost skirt was planned (thermally insulated foundation slab with PE foil against moisture). However, after further discussions between the geologist and the architect, I was advised to invest in a waterproof concrete slab instead. This is because—in the worst case scenario—the groundwater level could be several decimeters higher, causing the foundation to sit in water. Therefore, waterproof concrete is recommended. With a 10 cm (4 inches) higher slab, the additional cost is over €8,000.

The extra costs add up (of course), and I would appreciate further assessments on this topic if possible.

My first question:
What are your experiences? Is the slab actually at risk of standing in water, or is the geologist being overly cautious?

Second question:
If the concern that the slab could temporarily be submerged is valid, are there reasonable alternatives to a waterproof concrete slab?
Of course, I am willing to do and spend whatever is technically necessary to protect my investment long term. However, I am looking forward to any input.

Thank you very much and wishing everyone a nice weekend!

Regards,
Sebastian

I don’t think that’s too cautious. We are still experiencing drought in the Südheide. Entire lakes have dried up here. Once things return to normal, the groundwater level will surely be higher!

...there are already experiences with pile dwellings at Lake Constance.

I'm not a professional, but I think you don't have many options. If you find anyone who would build it differently for you, they probably won't take any liability in case water intrusion occurs. And honestly? It might never happen, but with all the extreme weather events, the water level could be that high next year. Do you really want to ask yourself then why you saved on the measures recommended by the inspector?

I completely agree, as I mentioned in my last paragraph. Savings can only be made here if there are no compromises regarding stability, durability, and warranty.

Are there any alternatives to the chosen waterproof concrete slab?

Thanks and best regards