The idea of the house. Financeable?

Age of the children 8 11 15 years

And no further charges.

Ok. Asked the other way around. How much do you want to spend on your house (what should it cost)?

That's the question, if I assume the ancillary costs here for the rented house (265€ per month), I could pay back 850€ monthly. How much money can I get from that and what remains in the end for the house because of the ancillary costs water telecom garden ..........

Sorry if this gets tough:
With €2500 and 5 people, you don’t build a house. Not without additional equity.

You would have to take out a loan of at least €250,000. The collateral will be enough for the bank, they will soon own an expensive plot of land including a garage. And €250,000 is already tight.
For the €250,000 you have to accept a monthly repayment of about €1000 + €300 in incidental costs that come with such a house. But then you only have a 1% repayment rate, which means after 20 years you still owe well over €100,000.

What if your children want to study? Or do an apprenticeship that is very poorly paid? You know that you are obliged to support your children financially? How do you secure this case?

Why does someone build a garage for €25,000 if you don’t even know whether you can afford the house at all?

Because the property is next to the apartment building. 250,000 thousand is already a big number (for us). If I understood correctly from other posts, that means about 200,000 house, the rest other costs?