Together to homeownership - rent-to-own possible

We (not married) are currently negotiating about a piece of land and want to build in the next few months.
Our plan would be that he finances and I pay "rent". But since I would not be an owner then, I originally wanted to co-finance. From a financial perspective, however, I can't do that. We estimate the construction of our bungalow in the Freyung district at 450K€.

450,000€ for land+house+additional costs is not much (so do you even get the desired house+land for that?). On the other hand, 450,000€ (or possibly more) is quite ambitious for a sole earner.

Is it mainly about securing the case of separation? Then you can also conclude a contract between yourselves and agree on a "compensation payment" for this case. This could then depend on the remaining balance and the amounts paid by both parties towards the loan repayment. It becomes more difficult if the woman, for example, works part-time due to starting a family and foregoes income, then you can no longer view it so linearly.

You can also get married, then any possible increase in the property’s value would be considered joint property and shared in case of separation.

I cannot agree with the financial involvement mentioned as currently valid; the amount is low, but after all, the OP also lives there. Children don’t seem to be involved yet? Then she isn’t paying off his house, but this is a usual amount for a normal rental apartment. And a couple would probably also share the rent.