With €4,200 net, a flat-rate living expense deduction of €1,900 applies (do you still have to pay any health insurances, leases, loans, or similar?).
That leaves a calculated €2,300. With that, you can finance approximately €550,000 at 5% interest/repayment. Maybe a bank would accept less interest/repayment, then you could afford a bit more (at 4% it would be €690,000). Whether you want that is another question.
There is no building land in Mülheim for that. At least €300 per square meter, probably rather 350-400. That is already the extended catchment area of Düsseldorf or a good macro location in the Ruhr area.
I would roughly estimate living expenses now at €1,700-1,800 – yes, an estimate, but we definitely shouldn’t get to €1,900. Currently, I put aside about 21-23% of my net income and then whatever is left at the end of the month. Sometimes more, sometimes less; sometimes it has even been only €50, but otherwise, €200+ is the rule.
If you are, of course, in these regions, maybe a renovation would be cheaper again?
The goal is ideally to have everything off your feet before retirement begins, possibly only very shortly afterwards.
I don’t want to lie, it’s all a bit discouraging right now.