Are there experimental aircraft regulations in Germany?
Yes, there are. Everything is regulated in Germany.

Your regular experimental aircraft would have to follow JAR 23 and the LBA has transferred the supervision and certification of experimental aircraft to the Oskar Ursinus Vereinigung (OUV). Before you begin work on your experimental, be sure to contact them and talk to an OUV assessor. This assessor will follow the progress of your work and will regularly check if all work is done properly. You need to prove that you have a heated workshop and document your work sufficiently (like keep samples of every resin mixture that was used for building composite parts). This assessor will also help with contact to experts if you got stuck or with the insane amounts of paperwork, which - this being Germany - often seem baffling and pointless.

You do not need a full certification but at least a load test to 100% design load. The aircraft will get a vorläufige Verkehrszulassung (VVZ; provisional airworthiness certificate) which needs to be renewed regularly. Also, operation of that experimental will be restricted to overflight of sparsely populated areas. But others have succeeded before, so good luck!