House construction financing: possible or too tight?

I see it the same way.

For the salary and ongoing costs, the overall financing framework is simply too high. I would plan and calculate with 40 sqm less for half the risk. Then you can certainly sleep more peacefully in the long run ;)

Well, if you don't question the 200k and consider it really secure, it's not a problem. Then you can also safely choose a low repayment and comfortably refinance the remaining debt after 10 years. Without the 200k, it's quite tight, although the question is whether the partner's 1000 euro income will remain permanent.

Yep, the 1000 euros stay as they are because only part-time employed due to children :)

Income: 4800 Expenses: 3150

1650 remain.

Possible rate: 1600 - 1800 Euro

But the additional housing costs are still missing, which will be higher than before. With 1800 you run a loss every month! So that definitely doesn't add up... child benefit or not...

Including interim financing, you would have to take out another 800,000€. Then in the meantime there would be a total of one million euros in debt!! Definitely too much for 4800€ monthly income.

now to my question how much budget can we still afford with our data for the costs of the new building? Does this add up or is it too much?

I think it will still be possible if you first sell the current house (sell now, move out in a year or so) and then use the equity to start the new project. Is the house to be sold for 200,000€ already in your possession? Sounds like an inheritance to me. Or is it about a tax-free sale of a rented property? Well, what you do not currently own in your name will not be recognized by the bank. At most the local bank could be convinced with it.

Make an appointment at the house bank and please report what they say.

With plenty of (money) gifts from the family, many things are possible, including such a project. Without them, it would of course never be feasible.

365k and vacant sale with the income very risky