Property purchase unmarried

That is absolutely no problem and the tax office will not contact you about it either. We also bought our property together and had ourselves entered in the land register at 50% each. Since at the time my then girlfriend (now wife) had her equity still "locked" in fixed-term deposits, I transferred the entire amount for the property at that time. She now, in terms of calculation, pays more towards the construction. In the end, the municipality also did not want two transfers from us but the entire amount at once. The notary and land registry invoices as well as the invoice for the property transfer tax then come twice (each 50% of the total amount).

The only downside: If one of you (which we absolutely do not hope for) dies, there are also no exemptions in the area of inheritance tax. In the worst case, her parents or siblings inherit half of your house or if you inherit, you have to pay tax on her half again. That is definitely not insignificant.

And not at a later wedding then?

Who the money comes from is initially irrelevant with regard to a gift.

I don't understand that. If one person pays for the property but both are registered as owners in the land register in equal shares, then a taxable gift exists in the amount of half the property share.

In the worst case, your parents or siblings inherit half of your house

Spouses are also not automatically sole heirs of each other (keyword: Berlin will).

Just lend her the necessary money interest-free. In the land register, you register yourselves 50:50; upon marriage, you can then tear up the contract. You don’t calculate in a petty way.

If you separate within 10 years, she has to pay you this amount.
This can be put into a private contract, or other conditions or modalities, which both sign and both get.