@ypg: This passage is not included in the PDF version of the thread creator. Still, I maintain that the additional costs must be paid separately based on the building regulations in general. That's what the contract says, not the construction service description.
Yes, I saw that too. But I am exactly of your opinion. Anything else makes no sense.
I think you are wrong with that. He stopped the work, he did not comply with the contract (and did not terminate it either), but of course everyone can have their opinion.
Dear OP, somewhere in your contract
there will still be a point stating that construction will be stopped immediately by the contractor if the client defaults on payment. You recognized construction defects (you did not have a building surveyor yet, right?), withheld payments partially, and thus did not fulfill your part of the contract. Please check again where and how this is stated there.
The contract is missing the point 1 Zug um Zug. You have it. You can tell.
Otherwise, one has to say: we were not involved. Neither in your conversation nor in other situations. We also did not monitor the construction progress. However, I am surprised that you put all the blame on him. Ok, I cannot and do not want to judge construction defects, but it simply does not work to only pay a partial amount. Even if many (here) advise that: it is clearly regulated in the contract. At least that was the case for us, and it will be the same for you. When you reflect on many things afterwards, you often have to admit some partial fault yourself. Somehow that is missing here – and by now I believe you not only did a lot wrong but very much wrong. And you are still at it.
And you should not confuse opinions with facts.
Did you put the construction defects into writing with a deadline, or did you just say it and withhold money?