Building in existing structures - Subsequent additional costs despite a fixed price

Hello dear forum,

I am currently building a single-family house on an existing basement. At the beginning of April 22, I signed the offer at a fixed price. Five months after signing the contract, I was informed that the existing foundation cannot support the load of the new house and therefore must be reinforced. I am supposed to bear the additional costs in full.

I have the feeling here that the general contractor did not carefully examine the planned project and now I have to pay for his mistakes.

Already five months before signing the contract, the general contractor received from me the complete static calculation of the existing basement including the structure of the existing foundations, as I wanted to make sure that the project could actually be implemented. The one-and-a-half-story prefabricated house from the late 60s was supposed to be replaced with a solid construction (Poroton) with 2 full floors. Obviously, despite the available documents and site inspection, the general contractor hardly or not at all dealt with the statics until the contract was signed and promised us everything, as long as we signed the contract, and would look more closely afterwards (at least that’s how it feels now).

According to our contract, the fixed price includes the "raising on an existing basement." There is nothing about "possible additional costs," "a subsequent feasibility check," "subject to final statics," or other fine print.

The only room for interpretation I see is in the general terms and conditions. For example, it states:
"Any additional costs due to [...] circumstances for which the client is responsible [...] are not included in the fixed price." - but I do not see that as applicable here.
- Or do you have differing opinions on this?

Or
"The client is responsible for the ground conditions." - here it presumably refers to the property and not the existing structure, so that wouldn’t be a loophole for the general contractor either.
- Or are there differing opinions on this?

I am of the opinion that the general contractor made false promises here or did not sufficiently warn me. Therefore, I do not agree to bear 100% of the additional costs for the foundation reinforcement myself, or am I just unlucky as the client?

What is your assessment?
What do you suggest to me?
Is this my bad luck or bad luck for the general contractor regarding the costs for the reinforcement?
What would be a fair solution here?

If any details are missing for evaluation, I will of course provide them!
Thank you very much in advance for your support
Berlinho

I am currently building a single-family house on an existing basement. At the beginning of April 22, I signed the offer for a fixed price. Five months after signing the contract, I was informed that the existing foundation cannot bear the load of the new house and therefore needs to be reinforced. I am supposed to bear the additional costs in full.

April plus 5 months is September. What has happened since then? At what stage in the construction process are you, and is there possibly the option to change the builder?

On what basis does the general contractor justify his claim? If he relies on the previously known structural calculation (mentioned as known in the contract?), in my opinion it will be difficult for him. If he relies on new information, it will be difficult for you. But this is certainly a case for a specialized lawyer.

Addendum: Just looked at the contract plan. It explicitly states "Basement floor plan based on the existing plan of Mr. XYZ dated XX.03.2022 and execution documents for 'House type 104'." - House type 104 was the Neckermann prefabricated house built at that time. The contract plan therefore explicitly refers to the historical execution documents I submitted. - That should support my argument, right? A change is probably no longer possible. Demolition starts next week. From April to September, the building permit was prepared, with a few change requests (windows, etc.) discussed without apparently conducting a structural calculation for the first time. It actually took from September 22 to February 23 to receive the offer for the foundation reinforcement. There were always reasons why the offer was not yet available, because of course we were extremely interested in the costs.

On the topic of "new information": Two core drillings were carried out in the basement in August/September 22 (after signing). The background was that the general contractor had hoped that the foundation would be broader than documented in the historical records. They had apparently finally carried out the structural calculation and found that the foundation was insufficient. During this core drilling, it was found that the foundation is exactly dimensioned as indicated in the historical records. There is no indication in the contract documents that another core drilling or anything else would be necessary to guarantee realization at the agreed fixed price.

It was (allegedly) also found during the core drilling that the load-bearing, central middle wall (the backbone of the house) stands eccentrically on the foundation. My suggestion to simply rebuild the wall was rejected with the argument that even with a central position, the foundation for the planned project is undersized and definitely needs to be reinforced.

When I think about it, the issue with the "eccentric middle wall" might also have just been an excuse to shift the blame...

What does the contract say regarding the structural analysis / the existing condition?
If it states that the offer is based on structural calculation X on the basis of client data according to document Y, then the construction company will have a hard time getting out of it.
However, if it only says "Building on existing basement, glanced at it before," I see your cards as worse.

The contract specifically states:
16. Extension on an existing cellar - The fixed price includes the extension on an existing cellar.
17. Outer wall - The fixed price includes an outer wall [...] thermal insulation.
18. Inner wall ground floor/upper floor - The fixed price includes the inner walls made of sand-lime brick according to static calculation.
- Nothing more is stated.

What are your arguments/reasons that with such a wording the problem can be shifted onto me, or is that just a feeling?
This is not meant to be rude at all, but I am really looking for somehow reliable indications so that I know how strongly I should discuss the topic with the general contractor...