Hello,
I also thought long and hard about whether to hire an expert or not. In the end, I decided to do so (through the Bauherren-Schutzbund) and it is one of the best decisions of the entire construction project, and we are only halfway through! The shocker was the roofing, where broken tiles were glued with acrylic. They would have soon fallen on our heads. I expect something between 4000-5000€ for the construction supervision by the Bauherren-Schutzbund, but it has already paid off. No construction is free of defects. Of course, no construction company wants an expert, who wants to have their finger pointed at. But I wouldn’t let myself be fobbed off with such a reason and would rather wonder why the company is so opposed to an expert... Best regards,