Construction costs per square meter - how to calculate?

Please do not underestimate personal contributions...
…I have been chasing my brother-in-law for weeks to help me with the terrace railing, which, by the way, he offered on his own. Not critical, but somewhat annoying.

That was the reason why I only contributed personal efforts where I could manage it alone:
- Closing conduit slots
- Painting work
- Sealing window sills on the outside
- Cleaning work
- Cosmetic touch-ups here and there
- Excavating here and there

Everything else was not worth the hassle of coordinating time slots within my circle of friends.