I would like to provide a positive example:
Solid architect-designed house KFW55, gable roof, air-to-water heat pump, controlled residential ventilation, wooden windows, partially basement, 200m2 solid wood parquet.
Total living area 305m2, heated 270 m2.
Double garage 9.6x6 m.
Architect's services phases 1-5, partially 6+7, all individually contracted.
Start 06/20, move-in early 09/21.
The construction costs are pretty much exactly on the architect's estimate.
The contract for the shell was 30k under budget, plus the 3% VAT brought another approx. 12k savings. The additional costs for the budgeted plastic windows were easily manageable.
Including the garage, I am currently at around 2,250 €/m2 plus approx. 50 in own work.
We also approached various general contractors at the beginning. Leaving aside the poor designs, we would certainly have paid about 15% more with them.
You need a good architect, time and a network.