Land and house always belong together; they cannot be seen separately. So if SHE alone is listed in the land register, legally the house also belongs solely to HER. There is no differentiation like "the land belongs to you, the house belongs to both of us." Are you aware of that? That you don't own any part of the house as long as you are not in the land register? That remains the case even if you sign a contract with a construction company, are on the loan agreement, or sign the building application. That doesn't change anything. As long as you are not in the land register, you do not own any part of the house. In case of a separation, however, you would still have to continue paying off the loan for a house that doesn't belong to you. That's why it would never be an option for me to invest my money and my passion into a house and commit to a loan agreement if I legally have no claims to this house. Marriage does not change this fact either, because even after getting married, it still remains solely her house.
Ultimately, you need to know what you are doing... but for me, this arrangement would not be an option. Either one person is alone in the land register and thus also alone on the loan agreement, or both are together in the land register and on the loan agreement. Either or. There would be no other option FOR ME.
Whether it is possible to agree notarized that, in case of a separation, the land goes back solely to the girlfriend/wife, I don't know. But I would think carefully about that too. Because that would also mean: in the event of a separation, you would still be on the loan agreement but no longer in the land register. Which would also mean in this case that you have to pay, but get nothing in return.
What would you think about the arrangement that your future parents-in-law GIFT their daughter HALF of the land and you BUY the other half from your future parents-in-law? That would be the solution for me. This way, both of you are owners of the house and land, and your future parents-in-law have not given you anything they didn’t want to. How you then arrange the payment, whether you pay it out of your own capital or include half of the land costs in the loan agreement (which would mean your girlfriend would indeed pay for part of the land (loan portion + interest on half the land)) or whether the future parents-in-law give you a private loan for the land costs (so you effectively pay nothing at first) but you pay a certain amount monthly to the future parents-in-law until the land is "paid off", is up to you.
Or you really do it so that she alone is in the land register, but in return, she is also alone on the loan agreement. Then the lines are clear, it is HER house and she alone pays for it. You could then possibly pay rent to her monthly. This way, in the event of a separation, you cannot be held responsible for the payment of a house that does not belong to you, but you still support her in paying off the house through your "rental payments." The only question would be whether the bank agrees to this, whether your girlfriend has enough income to manage the loan alone. And it would need to be clarified whether you still contribute any existing equity. If so, definitely arrange notarized that you get that back in case of a separation.