Network cable defects

Yep, it still needs to be clarified exactly what is stated in any contracts.

Chiseling open the wall only comes into play if the BU did not install the cables in empty conduits.

What you are saying boils down to this: You order a car with electric window lifters. The manufacturer is too lazy / too stupid / too clueless to install them and simply installs manual ones instead. And you are supposed to accept the manual ones and even pay for them? And the manufacturer/construction company gets away with this in court?????? Are you serious?

r
conduit and DIN: just because something is stated in some DIN does not mean that anyone has to comply with it. unless, of course, you have agreed that this DIN is contractually binding.

I’m not very familiar with this.
As I read it, it was agreed verbally (and presumably discussed with the site manager) that double sockets were to be installed. But only single sockets were documented in writing. That’s how I understand this post at least.
The electrician presumably followed the written order.
The site manager verbally confirmed the defect to you.
If the construction company stands its ground and refers to the written agreement, in my opinion, your chances regarding "defect rectification" are rather poor. Whether the site manager’s memory (as an employee of the company) is still that good in such a case, I don’t know.

What I want to say is: from my point of view, it comes down to goodwill. The site manager is aware that a mistake was made. But according to the contract, the work was done "correctly." Therefore, the site manager will try to resolve this on a goodwill basis (as long as it does not involve very great effort). If you push too hard (i.e., demand major effort like breaking open walls, re-laying cables, etc.), the goodwill attitude could change.

But maybe I misunderstood the original post or maybe the contract only mentions the "socket" and the type of socket is defined elsewhere.

As I wrote above – I can hardly judge.
But as the original poster wrote, the contract only states one socket and not a double socket. He assumed it was the double socket he wanted (and discussed).

If there is only ONE socket in the contract, then the contract is fulfilled. (Among business people, oral agreements also count as contracts – but between private individuals and businesses, only written agreements count, if I’m not mistaken.)
That’s not to say the house construction company is not willing to be reasonable or to make concessions – after all, this business partly relies on recommendations. But if you come in with a heavy-handed approach (YOU MUST break open and re-lay), it can happen that the company thinks in terms of effort vs. benefit and simply walks away.