Briefly for comparison, my build 2021-24:
211sqm + 25sqm storage room also within the thermal envelope
Base slab without basement, 2 full floors with BSH ceiling, cold roof
KFW40 standard as timber frame construction
Brine geothermal heat pump with trench collector
Central controlled ventilation system
Electricity in KNX
25 kWp photovoltaic system without storage
Wood-aluminum windows with external blinds on the ground floor and roller blinds on the upper floor
Ground floor completely travertine, upper floor parquet
Wall-mounted fittings, freestanding bathtub and all the frills.....
etc....
When it comes to materials, we did not skimp at all, for that I can count the craftsmen who were here on one hand (I estimate honestly at least 80% own work). But that was only possible with 2x parental leave totaling 7 months, no double burden from rent, construction site within walking distance to home, and a very patient wife.
If I don’t include kitchen and outdoor facilities in my Excel list with over 600 individual invoices, I come to construction costs of about 1800€/m²
150m² + basement (or underground garage) + significantly more expensive outdoor facilities due to the hillside location..... never ever for 350,000€!
For that, you would really have to do absolutely everything yourselves and then also save massively on materials. Don’t let yourself be f***ed.