But the financing only works with her, it won't work alone (according to the advisor).
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I am receiving equity from my parents amounting to €50,000. Furthermore, my grandmother would support us with regular payments so that we can take advantage of the special repayment.
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Is there also the possibility to split the shares in the land register for example 1 to 5?
The nasty thing is always that the girlfriend only owns 1/6 of the shares, but like you she has to be liable for 100% of the financing amount. Not so cool.
Regarding the separation: If you can't manage the house alone now, of course the money is missing to "buy out" the shares or to pay her off. Additionally, the bank does not release the lady from the contract despite separation. I think as of today I would only enter such a contract from the woman's perspective with a clause that in the case of separation it MUST be sold so that she is released from the loan. Or 50:50.
I haven't read a really good solution here in the forum yet. What seems fair today in relation to contributed equity becomes inevitably unfair after raising children at the latest. Strictly speaking, the woman then does not earn money. The argument with children has already been made. But maybe the woman will eventually earn more than you.
€50,000 is a lot of money, but in relation to houses also relatively little. Despite this difference, I would not make a big deal out of it and simply divide the shares 50:50 classically, even if you are effectively giving away €25,000 to the woman. You could voluntarily do without the grandmother's special repayment (how high would that be?) and wait for the inheritance. The inheritance is excluded from marital gains even within marriage, so it stays with you.