Earthworks... Dispute with the occupational disability insurance

A surcharge of 4500 EUR seems really reasonable to me. It was significantly more expensive for us, even though we had less than 1 m of replacement.

I would have had to provide a compressive strength certificate when doing it myself, which would have eaten up any potential savings.

Wouldn't you have also been credited for that? It is definitely required, no matter who carries out the replacement.

Yes, that was included. But you know how it is when GUs credit something...

I had no landfill costs, the mounds are still lying around here.

I wouldn’t necessarily get into a dispute over the BU. That seems pretty common to me. How many cm are included in the construction service description usually varies. I believe it was 30 or 40 with us as well. A soil survey was not included with us, we didn’t do one ourselves either, we got it from the developing authority and used the borehole points nearby. The soil is wavy anyway, not even a borehole directly on the property gives you 100% certainty. Of course approximately, but we took it as it came. In the end, a total of 80 cm had to be excavated, the additional cost was limited. About 3000 if I remember correctly.

The wording and the procedure I would also describe as usual.
This does not necessarily mean, however, that your fear is unfounded.
The earthworks are definitely a point that can lead to massive additional costs and where the general contractor does not always pursue the same interests as you do as the client. The general contractor naturally prefers to work with "his" earthworks contractor, even if another may be significantly cheaper for you (with us, there was a difference of around €6,000 between the general contractor's earthworks contractor and the alternative, which we could not use anyway due to interface coordination, etc., etc.).

After an expert opinion with us, our floor slab had to have more iron. Cost point 3500 (at that time).

Regarding distrust and the surreptitious addition of costs: such a thing is fraud, and a sovereign company cannot afford a criminal offense. Let's assume that you did not end up with a Haiopai company. Then there is trust and occasionally following up and asking questions is also legitimate.

Have the delivery receipts for the installed material given to you, then at least you'll know if the volume is correct. It can still be for another construction site, but it is already less likely.