Building a house financially feasible or a pipe dream?

Plans for the extension can be found here:

Since we live in there, we can’t significantly rearrange the space. Especially since almost everything in the existing house is new and I’m reluctant to tear everything up... The planned extension could initially be done almost without affecting the upper floor apartment, except for one breakthrough. In the existing extension on the ground floor, only an intermediate ceiling would need to be installed. Effort yes, but manageable. For possible damages, €15,000 is budgeted, which according to the contractor definitely will not be needed – but better safe than sorry.

The plan is still based on the situation of 2 children, so I’m also asking how to sensibly create space for a third child without letting the costs explode..

Feel free to take a look.

If I want to have paid that off in 30 years at an assumed interest rate of 2.5%, a monthly payment of €1500 would be due. So that's what I can probably afford - that would be 33% of the net income.

You only have an income of 4000 euros and at the earliest 4500 euros in three years.
That also means you are dependent on two incomes for 30 years!

My net income: €3000 ([Gehaltserhöhung 2017 steht noch aus, sollten 50€ netto sein, dazu kommt eine schon beschlossene Tariferhöhung von 1,5% im Januar, sind wieder 50€ netto])

With 13.5 monthly salaries, of which usually none are planned for, you can easily add €200-€300 monthly.

Child benefit currently: €384 - with a third child then €582

Currently my wife earns €450, which she will continue to receive until the birth (if a third child works out). After that maternity leave, roughly the same amount. Then parental allowance 50% for 2 years, parallel to the €450 job that she can and will continue to do, so then just under €600 net per month for 2 years, after that either already a 75% position (=> €900 net) or another year €450 and after that 75% when all children are in kindergarten.

So worst case: €3000 + €384 + €450 = €3834 monthly - of which €1500 is 39% of net income.

Planned case (nothing will start with building and loans before March 2018 anyway): €3100 + €200 + €582 + €450 = €4332 monthly, of which €1500 is 35% of net income.

Optimal case (wife works 75% again after the third child): €3100 + €200 + €582 + €900 = €4782 - of which €1500 is 31% of net income.

And again: I have been paying the €1500 every month for 5 years and manage very well with it.

Yes, we will be dependent on two incomes for 30 years then. Shouldn’t I expect that at 35? Especially since I believe I am well protected against other eventualities?

With 13.5 monthly salaries, of which normally nothing is planned, you can therefore easily add €200-€300 monthly.

But this is not done by the banks.

Optimal case (wife works 75% again after the third child): €3100 + €200 + €582 + €900 = €4782 - €1500 of that makes 31% of the net income.

Then just apply for the financing at that time. For financing, only the current situation counts. Sentences with "planned case" or "after that either" you can completely disregard in financing considerations, at least for the bank, the numbers are irrelevant. Therefore: don’t calculate here yourself but go directly to banks to explore the real possibilities.

In your situation, it would be sensible not to run to individual banks, but to consult a financial advisor/broker. They can much more easily identify the potential partners.

I thought banks don’t include [Kindergeld] in their calculations?

Just write down your total living expenses... somehow I’m missing expenses in my mind for which there won’t be any money left afterwards