Electrical installation by a friend - Mandatory inspection due to insurance?

First of all.

None of this is illegal (in detached houses). The Criminal Code is for other matters.

But yes, insurance companies can be difficult when there are faults in/from the electrical installation.

An E-Check by a professional company would be the correct way to protect yourself against that. Just ask a few companies. The effort for a proper E-Check should not be underestimated (access to every socket and every terminal); proper documentation beforehand helps massively.

None of this is illegal (in the detached house)

Then we simply say: it is not allowed. Period. End of story. There is nothing to discuss about it. It should be Paragraph 13 of the NAV. It's just like with water pipes.

Then we simply say: it is not allowed. Period. End of story. There is nothing to discuss about that. It must be Paragraph 13 of the NAV. It's just like with water pipes.

Even the electrical practitioner classifies NAV §13 in such a way that the obligation to register only applies up to the measuring devices. After that, a "normal" electrical specialist may take over.

But yes, the paragraph is probably (intentionally) formulated in such a way that it can be understood differently.

It's just like with water pipes.

I thought I am allowed to do everything myself behind the water meter. So, for example, connecting a PE pipe that goes into the garden to a branch specifically intended for that purpose is allowed, isn't it?

Regarding electrical work:
Everything inside the house has been inspected. Everything was done by the company commissioned by the main contractor.
For us, it's about additional work or everything that is outside and in the garage. Maybe I will actually find someone who will then create a test report for me. I mean, no one can understand any changes inside the switch box in the house later anyway.

Even the Elektro Praktiker classifies NAV §13 in such a way that the registration obligation only applies up to the measuring devices. After that, a "normal" electrician is allowed to handle it. But yes, the § is probably (intentionally) formulated that way

Sorry, but no one interprets it the way you do. It is unambiguous.

I thought that with water I am allowed to do everything behind the water meter myself. So for example, connecting a PE pipe that goes into the garden to a branch specifically intended for that is allowed, isn't it?

Drinking water is stricter than electricity!

You should definitely keep your hands off anything that is actually used as drinking water.

A garden irrigation behind a system separator/backflow preventer is more likely allowed. But I would have to google the purely legal aspect first.