Construction contract additional costs foundation reinforcement?

Hello, we are currently building a new house with a construction company, with a building application, drawings, structural analysis, and energy certificate included in the contract. BEFORE the contract signing, I sent him an email asking him to please check the plot of land, with a soil report attached. After the foundation slab was completed, he informed me that the slab would need to be thicker. Then an email came saying

"
We assumed that the soil conditions were normal before the contract signing, but after inspections by our structural engineer, it turned out that poor

soil conditions are present.

Due to the poor conditions, the foundations must be reinforced and the slab must be made thicker, from 18cm to 30cm.

Unfortunately, this results in additional costs of €5,600.00 including VAT, we ask you to confirm this.
In the additional cost statement it now says 5600+19 VAT....

In my building contract it says

"Foundations and slab"
Concrete and dimensions according to structural calculations. The slab is a concrete slab with appropriate insulation that complies with technical regulations. The foundation is carried out with strip foundations made of C20/C25 according to the requirements of the structural calculation. A galvanized steel strip is inserted into the foundations as foundation grounding. According to the structural analysis, a concrete slab is poured on an underlying construction foil on compacted fill sand. Further back in the contract it also states that the client (owner of the property, so me) also bears all incurred fees such as rough construction approval, TÜV, chimney sweep, as well as any additional costs for structural engineering, architect etc. incurred during construction.

Now the question is, are these additional costs justified? Do I have to pay them? Before the contract signing, he or the architect should have seen this.
The contract does not mention an 18 cm slab...

Thank you

The contract states: "Concrete and measurement according to structural calculation" - this means that it is a variable item. The structural calculation requires "more". Thus, the variable changes and the price increases. From my point of view, this is to be paid.

The question of whether the architect should have seen this beforehand is obvious. If the soil conditions in the area are generally considered "difficult", he could have addressed it. Of course, I do not know what was discussed and understood about variable items in general and here in particular. In terms of price, it would have made no difference.

The amount seems quite fair to me - without knowing the quantities exactly - and by no means a "rip-off approach". I would not make a fuss about it - but take the incident as an opportunity to mark the variable items in a meeting and note the cost risks. That gives you security - or at least certainty. At the same time, you develop a culture for subsequent communication. Getting through a potentially critical situation well from the start is often a blessing for the project’s progress.

In this sense: keep your head up and have a good construction phase!

Maybe you can still negotiate a bit. After all, you handed him all the necessary documents on time. I would say that this was not accounted for in the financing and try to meet the contractor halfway.

Structural analysis is always initiated only after the contract is signed as part of the detailed planning, which is why the contract always includes this addition.

I don't think you can do much about that. The previously sent soil survey won't change much, unfortunately.

What does reinforced mean? Do the strip footings have to be dug deeper? I would still negotiate a bit with the BU. Because just thickening the slab from 18 to 30 cm won't cost him 5600. It probably doesn't even cost him that much for all the concrete for the slab... That's why you should always have the structural analysis done beforehand, then there are no more surprises

Hello,

due to the soil conditions and moisture in the ground, the floor slab also had to be reinforced in our case. In addition to the waterproof concrete, reinforcement of the basement sealing using mesh was also necessary. Everything was calculated as follows: