400k house financing - Who can take away my fear?

You are by no means too cautious. We are also spending about 500k, but exclusively in cash. With 400 to 500k debt (everything only ever gets more expensive during the planning and construction phase, really!) I couldn’t sleep a single night peacefully anymore. But we are also a bit older... so the situation is not directly comparable. Two children will certainly cost you - until they are financially independent - about as much as 2 new Ferraris, personal experience. So there won’t be much left for special repayments etc. Better save for a few more years first, as long as there are no children yet. Quite a lot should be possible with a net income of about 5k. Also, the construction boom - and thus the price boom in building - won’t continue like this forever; in a few years everything might become a bit more relaxed and therefore cheaper again. There is strength in calmness...

32/34 years old: 400,000 EUR financing with 5,200 EUR net salary + holiday/Christmas bonus/bonuses and 220,000 EUR equity + reserves = What are you waiting for?

With us, it is by far not such a high amount, but would you rather pay a bit more interest and the same for the next 20 years. Otherwise, it is too much of a gamble for me.

And how many will cancel the contract after 10 years and sign a new one? Probably some

As Joe already said, it’s a gamble, one wins, the other doesn’t.
But I would like to point out: with a 20-year fixed interest period, many banks already fall through the cracks.

I did not plan for any special repayments at all. If it doesn’t work out, you’re left stranded contrary to the plan. I then simply set 3% repayment so that I don’t have to finance as much with the follow-up financing in ten years. Nevertheless, 7% p.a. special repayments would be possible.

If you then also plan for 450€ additional costs, you are spending over 2000€ per month just for the place. I only calculated with 2% repayment, which means the monthly burden is lower, I am more flexible, and I am still below my psychological pain threshold of 1800€ p.m. However, I have to make more special repayments for that.