question: what do you still hope to achieve here?! you already have an expert and a lawyer involved. if resolving this together at the table doesn't work, your lawyer will probably have to show you further options. that always takes time and costs a lot of money. as the construction expert already said, there are always 2 rounds and it ends in a settlement, so you will have to pay your legal fees yourself either way. in the end, in 2 years there will be a few euros from the settlement with which you can possibly, with a lot of effort, recover your own costs. the deficiencies (e.g., PE instead of metal pipe) are then basically "settled away." basically, you MUST somehow get out of this situation reasonably with the GU today. with all the defects/errors you have listed, however, it probably won't end well. in the best case, you get out of your contract now and can finish the construction yourself.