I also tried on the website. The expert search shows quite a few categories starting with "Bau". I tried a few with postal codes 33 or 44. I am shown experts from Berlin and other places that are farther away. So either the search is pretty bad or they don’t have a good selection of experts in the construction industry. If I only enter "Bau" as the subject area, I get a few better results, but then for example monument preservation, roofer, but no construction expert. Ultimately, you could call there directly, maybe that works better than the search, perhaps it is just outdated. Alternatively, I have already found something at the Bauherren-Schutzbund, the experts there uniformly cost €85/hour nationwide. You can also get non-binding information by phone in an initial conversation about the procedure and expenses there. However, I also want to contact TÜV and Dekra again to find out how it is with them.