AVBWasserV §12, Customer installation:
(2) The installation may only be constructed, expanded, modified, and maintained in compliance with the provisions of this regulation and other legal or official requirements as well as according to the generally accepted rules of technology. The construction of the installation and significant modifications may only be carried out by the water supply company or an installation company registered in the installer directory of a water supply company.
Edith: Before the question of a significant modification arises:
[*] Changes to pipelines through, e.g., threading, soldering, welding, screwing, clamping, and gluing
[*] Connection of installations and apparatus without DIN-DVGW mark
[*] Maintenance and replacement of safety or security fittings
[*] Replacement of a floor-mounted shower with a hose shower
[*] Replacement of a floor-mounted or wall-mounted fitting with one featuring a pull-out shower
I am repeatedly amazed, no, appalled, at how many people actually tinker with the drinking water network themselves (which includes the customer installation). Worst of all is this ridiculous crap sold to people at their doorstep, which does not even have DVGW approval. When the water utility is looking for someone to blame after a drinking water network contamination, the air quickly becomes thin for do-it-yourselfers.