Proper piping system & appropriate diameters for sanitary?

As I said, you are not allowed to do that and no one can help you with it.

Is he allowed to change the tires on his car as a social worker?
Or is that only allowed by a trained automotive mechanic?
A moving vehicle poses a significantly higher risk to the general public.

He is allowed to change his own wheels.
What one has to do with the other is not clear to me, or was that simply a question?

By the way, there is the "Verordnung über Allgemeine Bedingungen für die Versorgung mit Wasser", it is stated there. I do not know of any regulation regarding the handling of winter tires.

By the way, there is the "Regulation on General Conditions for the Supply of Water", it simply states that there.

Sorry, I’m not an expert. Where exactly?

By the way, there is the "Verordnung über Allgemeine Bedingungen für die Versorgung mit Wasser", it simply says so there.

Applies to the public supply line...

Up to sealed meter.

No, §12 applies to owner-occupied homes.
- In paragraph 2, who is allowed to make changes to it.

Strictly speaking, you are not even allowed to change a faucet - but one shouldn't exaggerate either.