Deficiencies in the new construction. Dispute over the amount of withholdings.

The 500€ are intended as a compensatory payment, as the developer indicates that carrying out this work would be disproportionate.

I asked myself the same. And also where your expert has gone all of a sudden. He knows the damage pattern.

There has been no expert report so far. Only the assessment of a friend who is a sanitary and supply technician. The expert report is to be created because we do not agree on the amount of compensation.

Just a quick question for understanding: Do I understand correctly that you are asking your counterpart for the money it would cost to reopen the entire shaft in order to replace steel with copper?
Does the steel have significant disadvantages like rust, for example?

Copper is more corrosion-resistant and has lower pressure losses due to pipe friction than steel pipes. However, steel pipes are more susceptible to corrosion. For example, they are not suitable for areas that are at risk of moisture.

I could also insist that the copper pipes - as contractually agreed - be installed, couldn't I? It's my right to receive what I have paid for or what was agreed upon. Or am I wrong?

It's not that simple. You could (and should) have insisted during installation that copper pipes be installed according to the contract. Afterwards, it depends. If the installed material is unusable for the intended purpose, it will likely have to be replaced. However, if the material is common for that purpose (just cheaper), you cannot necessarily demand a replacement; it depends on proportionality and the scope of the work. If you cannot assess or quantify this yourself, I would suggest involving an expert who can financially evaluate the defects. Based on that, you can then talk with the general contractor.

It’s not that simple. You could (and should) have insisted during installation that copper pipes be installed according to the contract. In hindsight, it depends. If the installed material is unusable for the purpose, it will probably need to be replaced. But if the material is common for that purpose (just cheaper), you can’t necessarily demand a replacement; it depends on proportionality and the scope of the work. If you can’t assess or quantify this yourself, I would consult an expert who should financially evaluate the defects. On that basis, you can then talk to the general contractor.

No acceptance of the individual trades was offered, so I could not check the pipes installed during construction. According to his own statement, the developer has accepted all trades from his general contractor. He should have had the duty to point out the error to his general contractor, shouldn’t he?

The developer claims to have accepted all trades from his general contractor. He would have had the duty to point out the error to his general contractor, wouldn't he?

Yes, of course he should have. That is why he is also obliged to carry out defect rectification or reduction. I just meant that as a client you don't necessarily have to trust blindly and usually also check yourself if everything is correct, unless you also bring in a construction supervisor during the construction. It is of course a different matter if you were not allowed to enter the construction site, I have no idea how it is with developers.