Additional positions in house construction: are they useful and reasonably priced?

And did you notice a big difference between central and decentralized?

Clear yes and no ;) .

The central ventilation can only be noticed in a completely silent house if you know it is there (except at a higher performance level/party mode). The decentralized ventilation is noticeable. For me, it runs on basic ventilation, which is sufficient for me (six units for 120 sqm). Noise level is not noticeable when normal life is going on in the house. If it is quiet, you hear a soft rustling. As soon as you use or have to use a higher level, it becomes more noticeable - I would not want full power as continuous ventilation.

In new construction in this order: 1. central controlled residential ventilation 2. central controlled residential ventilation 3. decentralized controlled residential ventilation 4. decentralized controlled residential ventilation 5. decentralized controlled residential ventilation 6. manual ventilation

The thing with the drywall installer is a good thing, can someone else confirm that price-wise?

I can confirm that: drywall installer or plumber does that

1. Compensation for fittings and sanitary objects

these sanitary objects would be included:

The question to ask is: for how much added value in appearance/quality do you put yourself through this?

3. Installation of a secondary entrance door as a wooden door into the utility room

is a floor plan question: I hardly know any house designs that require this exit door. Either the utility room is too small or there are other routes one takes more readily

7. Installation of an additional shower in the guest WC as a flush-floor, tiled version

with a shower enclosure I would take a tub. Why bother with an expensive flush-floor when it’s going to be separated by a rail anyway?

10. Installation of an additional cold water connection in the ground floor WC next to the toilet (for the later on-site installation of a bidet shower)
Additional cost: €290.00

why do you need a bidet in the guest WC? Or is it only about the shower?

12. Installation of a laundry chute from the upper floor bathroom to the utility room (ground floor),

For one story I consider a chute unnecessary.

well. You have to burn through €17,300 first

… says someone in 2022? Politics have been asleep for 10 years, so now we can waste energy?! Is this actually a subsidized house?

Operating the forum with a mobile phone

For me this forum is realistically only operable with the phone (Safari). Everything else is rubbish.

Just a quick note about the deep boxes: Aren't you already making an appointment with the electrician to decide on switches/sockets etc.? Of course he'll install something there again. Either he simply makes you deep boxes or you tell him afterwards that he convinced you to take the deep boxes. That way you might still have a chance to save something. And just because I notice it’s not on the list: clarify early on where you might need 5-core cables.

(...)
… says someone in 2022? Politics has been sleeping for 10 years, so can we also waste energy?! Is this actually a subsidized house?
(...)

I thought so at first, too. But if you calculate with an annual performance factor of 3 and 50 cents/kWh electricity, then for 17,000 EUR you can burn 3,400 kWh more for 30 years before you break even with the ventilation system. And the 17,000 are financed anyway; he doesn’t have the cash lying around. Especially if at some point photovoltaics are added to the roof (or are even already planned), then this central solution doesn’t pay off with that surcharge even in 100 years.

In this respect, I agree that he will forego this expensive fun and instead aim for the decentralized variant for a fraction of the price.

But I’m happy to be proven wrong. How much heating energy in kWh/m²/a does a central ventilation or a decentralized ventilation save compared to classic airing until the humidity is back within the limit?

Just a quick note about the deep boxes: Aren't you going to make an appointment with the electrician anyway to finalize switches/sockets etc.? Of course, ask again what he installs there. Either he simply installs deep boxes for you or afterwards you say he convinced you to take the deep boxes. That way you might have a chance to save something. And just because I noticed it’s not on the list: clarify early on where you might need 5-core cables.

And always remember:
The craftsman prefers to do a direct deal with you, bypassing the general contractor/developer. Because he can charge you higher hourly rates and margins, which are still significantly cheaper for you than going through the general contractor.

All tilers I spoke with wanted to build the niche completely themselves. The drywall installer should only mark where the stud frame is located. Makes sense to me too, because the tiler can best judge exactly where and how big the niche should be to make the tile pattern look good.