Saving or building, which is more sensible?

You are probably right about that.
I still hope that I can manage it in 5 years.
By then, my income should also increase somewhat.
And whether I really need the mentioned construction amount is also uncertain.

I am fully aware that it is not wise to manage everything alone, of course it has advantages and disadvantages.
Maybe I also think too complicated.
The main reason why I want to manage it alone is focused on only one scenario.
WHAT IF THERE IS A DIVORCE??
If I manage it alone, there is no stress.

WHAT HAPPENS IN A DIVORCE??

In the event that an ex-partner keeps the house, their previously contributed equity does not form part of the amount to be paid out. The rest is marital gain.

(feel free to correct me here)

I don’t know how the imputed housing value is to be credited against spousal support.

But seriously: You avoid a divorce by not getting married; otherwise, you live with the risk.

If you want YOUR house, how do you make that appealing to a partner who, as in 99.9% of cases, wants YOUR house as a nest??

I consider your views somewhat naive, frankly, which at 25 is still fully okay.

Always remember: As soon as children are involved, you benefit from your wife’s UNPAID work in ways that are difficult to measure. What is her equivalent value if she is living quasi-rightlessly in YOUR house?

Show me a woman who does NOT want to marry. That's difficult.

If you want YOUR house, how do you make it appealing to a partner who, in 99.9% of cases, wants YOUR house as the nest??

Do you mean this in the case of a divorce or generally?

Maybe, I'm probably naive in that respect.
And yes, I am inexperienced in that regard. That's why I'm here to get advice.

Good question. I can't answer that for you.

I am really grateful for your advice. How would you approach the situation?

No, I mean that in the existing partnership a home that is felt to be shared should be created. YOUR house is not YOURS AND YOUR PARTNER'S house. Plain and simple.

What I would do: Don’t be afraid of theoretically losing something that I don’t really need anyway because of my attitude towards life. Then be clear about the life plan. Woman and child(ren) YES or NO?

If YES: Marriage protects the woman too. She will, in the vast majority of cases, give up career opportunities and workdays and support you in upbringing, which enables you to have a full-time job and thus income and asset building. That’s why she should be allowed to share in case of separation.

If I wanted a woman and child: marry, finance a house together with two incomes and plenty of buffer, then kid. With all risks AND opportunities. My opinion

I can only advise you to treat yourself to something once in a while. I believe living frugally does not mean denying yourself everything.

Thank you for your honest opinion.

I was also advised to work less and take more vacation (last year I had 5 days of vacation until Christmas) and I didn't manage it. I just find work very important. And working on weekends is the balance for my construction supervisor job. Basically, you are completely right, and I will try to put this into practice.