Construction report single-family house 1200 €/m²

I now have to pay just over €500 for my 3sqm. Is that expensive or cheap when building a house? I lost a healthy relationship with money during the house construction.

It would be worth €500 to me. Moss pictures are more expensive per square meter ... but also more decorative ... :) (and not comparable ...)

With or without plaster?

A controlled residential ventilation system is also more expensive than window ventilation, but it is widespread nonetheless (and probably also sensible) ...

You just have to weigh price against performance. Controlled residential ventilation has a completely different benefit.
In the bathrooms, I also have wall heating, which is more comfortable.
But throughout the whole house... 20-30k€ cost for saving 50€ in heating costs per year and hardly any other benefit.
People prefer to invest their often limited money in more sensible things, like controlled residential ventilation, or blinds, kitchen, etc.

Outside the bathroom it is also impractical, because you have to think carefully and measure well every time before drilling a hole in the wall or hammering in a nail. But outside the bathroom, the required heating power per m2 is usually much lower, so it doesn't make that much sense to cover the walls with heating. The main advantage I see is a warm bathroom without an ugly ladder.

...
The main advantage I see is a warm bath without an ugly chicken ladder.

... or as surface cooling in summer.

So I achieve 29 degrees supply temperature and an annual performance factor of 5 with an air-to-water heat pump for the [Wandheizung]. This is unbeatable in terms of price/performance.

But of course, if you have [Wandheizung] done by a company, that's a completely different story. It always depends on the individual case.
My colleague from the engineering office Heckmann, who did the design for me, does exactly such planning with which you can then approach the companies.

Plaster costs me 350 euros more because of the 15m2. Material [Wandheizung] 200 euros. Effort 16 h.

Due to the savings on the shell construction, I treated myself to the ceilings on the upper floor (drywall) and ground floor (concrete plastering) for 2500 € plus materials from the local Bulgarian.

He has already installed the vapor barrier so that it can be plastered next week. He insisted on sealing the foil to the masonry with glue and sealing tape.