Additional costs for sockets and network

Sometimes the developer allows you to coordinate and order such requests directly with the executing electrician. Then it is a bit cheaper

But everyone involved knows that they can rip you off like a Christmas goose.

Going directly with the electrician is not possible, because he could only come in after completion, that is, when all walls are closed.

Of course, you contact the electrician of the builder and generate a separate order with him! If necessary, on the day he is first used. Additional jobs are rarely declined

Best regards in brief

Additional deals are rarely rejected

2-3 years ago, I would have agreed with you. But in the current situation? A friend experienced 4 weeks ago that he only got 1/3 of his requests fulfilled. Time pressure... the team had to move on...

Oh....
The journeyman might be able to do that in overtime
(Nein, keine Schwarzarbeit - das ist etwas anderes )
Best regards in brief

Why should the BT allow that?

It depends on the builder (and the electrician). There are builders who let all additional matters run directly between the buyer and the subcontractor (here electrician). And there are builders who forbid this direct way* and have everything go through them. The direct way promises somewhat better prices, the "official" way facilitates acceptance and disputes about warranty issues and adherence to deadlines.

Our builder prices, valid in 12/2015, all gross
equal net, all including delivery and installation, tutto kompletto, switch series Merten M-Smart polar white:

[*]Schuko socket 35 € (2-fold times 2, 3-fold times 3, ...)
[*]Series switch 58 €
[*]On/Off switch 44 €

[*]Access point automatic switch IP44** 171 €
[*]Wall/ceiling outlet 20 €

Credits for omission

[*]Antenna connection socket 70 €

[*]Telephone connection socket 69 €

* They cannot prohibit the buyer, but their subcontractor.
** That should be the motion detector at the driveway.