Bank wants co-ownership of the property - experiences?

I advise you to handle everything on your own and let them live at the low rent. Sooner or later the circumstances will change anyway.

I am also a big fan of this option. Give feedback to the bank that the contract will only be signed by you. Or will the creditworthiness no longer be sufficient then?

Apparently, the girlfriend is somehow supposed to co-sign the loan agreement. You can't really tell exactly from the initial post. So if you've thought about repayments/payouts and whatnot, why hasn't this second private contract, perhaps also a loan agreement with the girlfriend (preferably notarized), been initiated by you yet? You don't sign a binding bank loan agreement without your own security.

EDIT: Words and thoughts are unfortunately also quite ephemeral.

We are supposed to finish that by tomorrow,

Just flipped back once more: I think some interpretation and explanation stuck after this bank conversation anyway.
Because the quoted sentence is not feasible. Sometimes it’s the individual, non-interchangeable words that change the meaning.
I can’t imagine that the bank said it _like that_.

By the way, there are many threads here that deal with one contributing more, the other less to the pot. At the latest after having children, the woman has contributed the most anyway – with accompanying disadvantages. That requires compensation.
… and with or without children: You keep the house, which was partly financed by her, she leaves without. So without taking anything from those years with her. The savings pot and retirement provision is only yours, while she will basically end up with nothing.

Crazy bank. She is right in what she says - without you having any agreement among yourselves - but I have never heard of such a condition in almost 30 years in the financing business. It's not a credit-related condition, but merely a moral demand/condition.

Such a condition was also imposed on us when we planned to move into and renovate my grandparents' house. However, we now have our own building plot, so it was not completed.

I would like to contractually agree that the money she paid in will be paid out to her in the event of a divorce.

That is exactly okay if you guarantee that you do not want to have children; even then, the capital paid in must still be interest-bearing. If children are planned, that is not possible at all. But there are still plenty of constructs that can be contractually regulated and that put your wife in a better position in the event of a separation.

She is reading along and I am discussing this with her.

She does not even want to receive it as a gift; she wants to have the part she contributes paid out later. The question is whether we bow to the will of the bank, I give it away and somehow arrange that I simply get the share of the property back after the separation and give her her money.

But it is still disadvantageous for her. If she had invested the money elsewhere, she would have earned interest on it.

The problem is that when building the house, you cannot separate between the land and the house. They are firmly connected, so a co-owner of the land is also always a co-owner of the house. There are certainly ways to contractually arrange a buy-back agreement, probably also via notary. But someone else can surely advise you better on that, we did nothing of the sort.

But even if you bowed to the will of the bank – you will never get this officially sorted to this day. To get her into the land register, it would have to be done notarized. For us, the registration took months.

I could imagine that the bank is opposing this because they classify it as immoral. The legal transaction would be obviously very disadvantageous for your partner, maybe they are afraid that it makes the contract legally contestable? I think parents who guarantee for their children are a different matter. Parent-child relationships do not just dissolve like that. In your partner’s case, she would guarantee for the entire loan without owning anything of the property. If things go wrong, that’s a personal bankruptcy for her. Even if you later draw up a note that she will get her paid-in money back in case of separation: a) where do you get the money to pay her out? b) what if you run off to Sardinia with your new flame without settling your contractual obligations in Germany? Sure, the scenario is absurd, but it illustrates that she would then owe the bank the full installments for a property of which she holds 0 ownership. Not every separation goes smoothly, and people are friendly and considerate afterwards. I have seen former couples fighting each other psychologically after divorce in the worst way, trying to manipulate the children, and damaging the ex financially as much as possible. That is not so rare.

I would never conclude the contract as you intend, that is financial harakiri.

We are currently in the process of applying for a loan and now the bank, after we waited two weeks for a response, is requesting that the partner/co-borrower

This is obviously not just about a guarantor, but about a co-borrower