New building with approximately 350 sqm of living space

Why do you need 350 sqm? That has to be paid for and maintained...
Own work costs a lot of time and energy.
The floor plans do not look like 350 sqm, there is no wow effect, best to go to an architect.
Consider whether you really need this space, better to build smaller, with less own work and not have to watch every cent.
I really can’t imagine that you can get this property for 500,000, with the outbuildings...

The floor plans do not look like 350 sqm, the wow effect is missing, best go to the architect.

And the areas of the corridors are missing. However, there are also 80 sqm of storage areas in 6 rooms (including the heating room). What is missing from the house at that size? - Charm - what does the exterior view including windows look like? - Chalet flair - does the original poster know what a chalet is? - Entrance with suitable staircase - Charming room layout - just a large living room is boring. With that space, how about a zoned living/dining area/fireplace room or similar. This also applies to the sleeping area with the bathrooms and the dressing room - Connection to the garden. There is only a building part missing on the left and the feeling of a four-sided courtyard farm emerges. The many storage rooms around the paved inner courtyard fit that well ...

I really can't imagine that you can get this property for 500,000, with the outbuildings...

Only if you can really do everything yourself and have friends available who work for free. Just the materials alone devour (even at wholesale purchase prices) a pretty sum - whether it is 135 sqm of window surfaces or the heating system. And the energy saving regulation does not make it cheaper.

At the targeted price, only contacts with the Mafia help.

he already wrote that they have realized such a project at that price within the family (if that’s true)

Family = Mafia, company = Cosa Nostra (or vice versa, I’m not Sicilian and may confuse the two) *LOL*

It's a pity that the OP doesn't comment anymore!

He is probably just submitting the building application. The floor plan is perfect after all.

Only a building section on the left is missing, and the feeling of a courtyard farm emerges.

With a suitable plot, a concept of a modern courtyard farm would really be exciting!

Garage / Children / Parents / Living each zoned in their own building.
Glazed transitions in a conservatory style ...