Development of the property - what are you allowed to do "yourself"?

You are allowed to bury the blue drinking water protection pipe yourself if it is required in your area (it is here)

It was different for us. The earthworks were done by a civil engineer, and the laying of all utilities including the connection by the city (or its subsidiary).

You may build the wastewater sewer completely yourself and connect it to the sewer system... Rainwater too... of course it must not mix with wastewater.

Only with the separate system. With the combined system, both go into one sewer. Here in our area, the connection may only be carried out by official civil engineering companies and must in any case be approved by the city. Afterwards, the obligatory tightness test must of course be carried out, which in turn may only be performed by certain companies.

However, the regulations vary regionally, so I would simply inquire with the local builders' association or similar... or just directly with the utility providers

That's right!

Wow, thanks for the info first! By regional, do you mean that it varies from place to place or is it at least consistent within each federal state? (By the way, we are [Niedersachsen])

Different from place to place