Main lines incorrectly installed

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Which expert? Do you mean they will hire an expert.
You apparently have little knowledge of DIN standards. DIN 1986 applies here.

YOUR expert! otherwise you'll just be brushed off easily. We have already built, and as an engineer, I know some DIN...

If you’re already going full throttle like this, you won’t have much fun. Better get yourself an expert before it escalates in the end.
Throwing around some DIN standards is useless if you don’t have the DIN yourself... and you definitely don’t have 1986 on hand...

and you definitely don't have the 1986 on hand...

I have access to the DIN. Professionally related. An expert or building surveyor has been commissioned.

On Thursday, I have an appointment with a building surveyor.

On Thursday I have an appointment with a building expert.

Good luck!