Electroinstallation too expensive - oversized or too expensive?

The fact that most Elis install a 16A circuit breaker regardless of what is connected at the back does not mean that this is correct, especially in a single-family house... usually the saying is "we've always done it this way"... that's what I meant from a safety perspective...

There are also other factors like the laying method, cable length, etc.

The 3600 watts do not always apply... sometimes it ends at 3000 or even less...

If the circuits are properly distributed, they can also be on a single phase without any problem.

"Normally," the electrician should also measure resistances at the end and check whether the breakers / RCDs trip. From my experience, the RCDs are very sensitive. I once connected a socket under voltage and only touched the neutral or PE, and the RCD already tripped...

"Normally" the electrician should also measure resistances at the end and check if the fuses / RCDs trip. From my experience, the RCDs are very sensitive. I once connected a socket under power and only touched the neutral or PE, and the RCD already tripped...

However, the RCD only helps you with fault currents. Even small potential differences are enough for it to trip when connecting or disconnecting. If a contact burns out, the RCD doesn't care much.

I also don’t understand why some people still lay 3 x 1.5 to the sockets; for me, sockets belong 3 x 2.5 PERIOD

And why? That’s nonsense... even my CEE16A has 5x1.5 because that is completely sufficient. Some is also strongly understated – almost everyone does that (which doesn’t simultaneously mean that it is correct).

As you say, it can be done. However, depending on the cable length and the number of connections (contact resistances), you can quite quickly no longer achieve the required values with 1.5mm².

That is something different and must of course be taken into account - but you generalized very much there. As a rule, the 1.5 size is more than enough, because it is still about a single-family house.

The actual values that need to be achieved I do not know; my electrical master is responsible for that, as well as for the acceptance. However, he said that on the longest cables and if there are still 7-8 sockets/transitions, it can definitely become tight. Especially when the contact springs are still new, the values can still be achieved, but 5 years later suddenly there is a higher resistance. The cable does not cost the world if you order it cheaply, so better a bit thicker than too thin and possibly test during the measurement. Installing a new cable is then significantly more difficult, even if you have laid empty conduits up to the sockets.