Construction costs per square meter - how to calculate?

Year of construction 2022, move-in summer 2023, Rhine-Main area, KfW55, no own work, no smart home, no fireplace, no photovoltaics, no special architecture (protrusions etc.), gable roof.

What I would call "deviating from the standard" here besides KfW55 is:

[*]central ventilation system
[*]living space increased on the ground floor to a finished floor height of approx. 262 cm
[*]basement as living space
[*]more electrical wiring, i.e. sockets and LAN ports
[*]floor covering above the often offered €40/sqm by general contractors
[*]2 bathrooms or 1 full bathroom and 1 shower bathroom, however we were able to purchase ourselves at a large online retailer here, which saved us several thousand euros compared to an all-in offer from the sanitary installer
[*]partly blinds

If you start with €3,000 in the north/east and €3,500 in the south/west or near metropolitan areas, you should have a good first (!) reference value for a standard house.

Current general contractor offer for a multi-family house in Krefeld. 4,600€ per square meter of living space excluding additional construction costs (balconies excluded).

I had hoped that construction costs might decrease a bit again. Seems not to be the case.

We will take care of wall and floor coverings, sanitary fixtures, and installation ourselves, we have experts in the family and among friends. We would also like to handle the electrical work ourselves (electrician in the family). We hope to save a few thousand this way.

I would not create a mess when awarding contracts by interfering with the architect's work. Discuss the regular tendering process with the architect, in which your brother-in-law and yourself will also participate. [...] I would consolidate construction coordination with the construction schedule, including all helpers and own work, in the hands of the architect. [...]

If the concert is to succeed, I would NEVER exempt myself or my brother-in-law from playing under the baton of the same conductor as all other participants!

We will take care of wall and floor coverings, sanitary fixtures, and installation ourselves; we have professionals in the family and among friends. We would also like to handle the electrical work ourselves (electrician in the family). We hope to save a few thousand this way.