Okay, it's certainly not an absolute must. But I still think it's quite good to know that I don't have to put up with every nonsense a general contractor/general builder comes up with. I believe many builders tolerate certain defects simply because they don't have money for a lawyer, expert opinions, etc. Many general contractors know this and therefore initially play hardball. However, when lawyer's letters come right after deadlines are set for uncorrected defects, the ball is played back. Many GCs then take action when they realize that the poor builder doesn't just swallow everything because he has to, but that he actually has backup. And: less aggressively: [Die Örag bietet auch kostenlose Mediationsverfahren an.] There are certainly cases where the relationship is disturbed by trivialities. And both sides don't approach each other anymore because they think they are being taken advantage of. In such cases, a mediation process with an experienced mediator can really help.
3500 EUR seems quite steep to me. Of course, it may be that high due to your construction sum, but for me it's barely half...