Is building a house realistic for us?

I believe him more than an anonymous and rude user like you. It certainly isn’t checked everywhere, and it depends on the credit and the purpose of use. It is a VR Bank in a town with <10000 inhabitants. Maybe it is done that way there. In our construction financing, probably nothing else was checked except the usual, although our construction financing advisor also said that every financing is viewed, evaluated, and granted individually. You wouldn’t just put a few parameters into a calculator and get the optimal construction financing.

You see, that’s why the VR banks are overrun and too expensive. Large banks have flat rates and also take a default risk into account. But only that way do you make money. And yes, that’s where Mr. regional banker can indulge himself and bring up the 3€ booking fee in DM.

I am not an expert, so this is not advice and is not meant to be.

Regarding financing:
I think it’s doable. If I were you, I would aim for a loan with a long fixed interest rate, at least 20 years. Insurance companies currently offer this for financing their life insurance policies, it’s best to get offers through loan brokers. If I were you, I might aim for a full repayment loan, then you will be on the very safe side in the future, at least if you see your jobs as crisis-proof. If there is a realistic risk of unemployment, I would be more cautious.

Do you know the site Zinsen-berechnen.de? Here you can calculate financing models under Wohnen.

Regarding house planning:
Yes, you can save money with own work, we did too and the result was/is good for us. Be careful, you can also greatly overestimate the effort involved here.

We also planned our house with family growth in mind, so many "small" rooms. But we also planned a redesign, e.g. the possibility to tear down many walls and possibly be flexible at some point.