Is it still possible to optimize construction financing?

For example, I was not aware that both salaries are taken into account, even if I would only pay

No, that’s not what I meant. The question is who becomes the borrower. If both of you are named in the loan agreement, then both salaries will be taken into account. If only you become the borrower, then naturally only your salary will be taken into account. But in both cases, your total expenses are considered because you live together. Meaning, the bank takes your €1,800 as income, but all expenses of both of you are offset, either using flat rates or your actual expenses. And then it can indeed get tight.

All in all, this still seems to be within the budget, right?

In the end, only you can know that. What I and my predecessors want to say is: be realistic and rather conservative about your income (the current, the future, and during your girlfriend’s further education), as well as about your expenses (which are usually more than you initially think, because you forget or underestimate things like annual costs, reserves for a car/devices, etc., or the costs that come with children). And just run through a few scenarios – what if…. Then you can assess which budget is really realistic and responsibly available.

Regarding the child question: it's not about a child costing less for you because you get everything, it’s about the fact that your girlfriend’s salary would partially disappear for some time.

And believe me, when there is a child, you don't just want to put them in used clothes

Regarding the question about children: it's not about a child costing money - with you less, because you get everything, it's about the fact that your girlfriend's salary would partially be lost for some time.

Less yes, but definitely not as little as the OP currently thinks. Because even if you get many things, you will always occasionally have the opportunity to have to buy things again (very popular, for example, shoes...) just like child seats, beds, furniture, etc.... It adds up quickly.

And believe me, when a child is there, you don't just want to put them in used clothes

The first years that's fine. But much faster than you think, children also have their own ideas about what they want to wear and you don't always want to deny them that just because the wardrobe is still full of clothes. And that doesn't only start at school age but much earlier.

These are a lot of pieces of information that I somehow still have to incorporate into the plans. By the way, are there books or checklists that one can use to approach the topic and not forget anything in the process? Then a more detailed overview of the expenses would be possible, which might be crucial for the feasibility in this discussion.

Maybe it would help you to make a non-binding consultation appointment at your local bank. They can best calculate for you how your current situation looks and could look (for example, our advisor once showed us how the situation could look during parental leave). Many banks also offer an insurance check (what is there, what is missing, are there possibly better/cheaper contracts). Remember that your insurance costs and also additional costs will increase with home ownership.