Why building a house almost always costs more than calculated

I suspect that you signed your contract later.

Correct. I signed at the beginning of April.

... was the start of construction or completion always planned for after December 2017?

Safety for wooden buildings does make some sense.

Then approach it differently and write to your construction company that you naturally assumed at the time of signing the contract that all offered prices always correspond to the required technical standards at the time of the planned execution. And this new electronics regulation was already known well before April 2017 and therefore is surely already factored in .

EDIT: ... addition .. and calculation errors should not be at your expense.

In April 2017, the standard had already been established for over 1 year.

The installation of these switches in single-phase alternating current systems with an operating current no greater than 16 A is mandatory for new buildings upon completion of the electrical installation starting from 18.12.2017.

Well, the ladies and gentlemen have put that nicely. Because it does not apply per se to new buildings, but especially to those from Weberhaus and other wooden houses.

In April 2017, the standard had already been established for over 1 year.

Can this be looked up somewhere, or can I prove it? In this case, I would definitely go up in arms. If Weberhaus also only became known in May 2017, I can somewhat understand that. Then only the poor communication to the builder remains, but if it was really already known in 2016, then I don’t see why they wouldn’t have planned that in for me directly. Especially with a manufacturer who builds exclusively using timber construction, such an omission must not happen.

It can be read on relevant sites. My recommendation is Elektrofachkraft.de