Recording payment flows of an unmarried couple

Hello,
My girlfriend and I want to buy a house. However, the financial contributions are very unequal.
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.
My girlfriend is not bringing in any equity.
But the financing only works with her; it won’t work alone (according to the advisor).

1. How can future payment flows be best documented so that in the event of a separation the ownership can be divided as fairly as possible?
I have read that for the equity amount it is best done with a partnership agreement. But how would future payments be handled? Would one have to make a new partnership agreement for the annual special repayment (if it is only made by one party)?

2. Is there also the possibility to split the shares in the land register, for example, 1 to 5?
What would happen in the event of a separation if
a) the house is paid off, and
b) the house is not yet paid off?
Could the person with the smaller share simply be "bought out"?

The contract that best solves your requirement is called a prenuptial agreement.

The contract that best solves your requirement is called a marriage contract.

For an unmarried couple - have I missed an innovation again?

He probably means they should just get married. Without knowledge of the personal circumstances, however, I consider this recommendation as "the best solution" to be bold.

Our own solution for a similarly situated problem: SHE pays a fixed monthly amount into an account of mine. SHE receives a co-ownership share that roughly corresponds to her repayments during the loan term. The joint loan (+ equity) is serviced from my accounts. Her payments are documented via the separate account. If there is a separation at time X, it is just a matter of calculation. Advantage: There are no problems with gifts and SHE benefits equally from the property's appreciation. And it is simply simple.

For us, it used to be an interest-free loan from my father: in case of separation, the money, i.e. the loan, would have to be repaid. This provided security towards the partner in the event of separation. Just as an example.

Since you come together (whoever owns how much money) to commit to life together, you should be aware that after 5/7/10 years or in the case of marriage or children, not everything is calculated exactly anymore. I would get advice from a professional, a family notary or something like that.

What would happen in case of separation,

The financing only works with her, it will not work alone (according to the advisor).

I read this as "in case of separation, the financing will fail". A "rent" from her seemed to me - however, I am not a financing expert - more sensible than adding up her income.

2. Is there also the possibility to divide the shares in the land register, for example, 1 to 5?

A partial ownership of a property can certainly also be 815/4711, even if there are only two partial owners.

He probably means they should just get married. Without knowledge of the personal circumstances, I consider this recommendation as "the best solution" to be bold, however.

For the same reason, I have also refrained from this suggestion - a property company does not have to pursue other purposes of the same partners (Tisch & Bett GbR aka marriage).