Initial questions about the topic of house construction

I don't want to give life advice now, but I can try to contribute a little to clarification (among other things).

[*]A distinction is generally made between possession and ownership (a book from the library is in my possession, but the library is the owner. I am the possessor of the rented apartment, but not the owner).
[*]Houses are always an inseparable part of the land on which they stand. The owner of the land is always also the owner of the house. The owner of the land is only the one who is registered as such in the land register (this can also be partial, e.g., 50% father-in-law, 50% mother-in-law).
[*]Parents are free to give a plot of land to one child, even if there are other children. Only if inheritance occurs within a certain period could a revaluation take place. Family aspects are excluded (discord, envy on one side -- cash payment under the table on the other side).
[*]Unmarried people are legally strangers. Without a (rental) contract or land register entry, there is a realistic chance of ending up on the street with nothing in case of separation.
[*]In married couples, ownership is not automatically acquired, but in the event of a separation (only then!) the accrued gains are divided.
[*]Gifts to a spouse (such as the parental house) can remain disregarded in the accrued gains compensation --> Concrete advice would be necessary here.

The cleanest way would be:

[*]Parents' land is divided (--> notary as point of contact)
[*]Partitioned land is gifted to the son
[*]Son builds a house on it with you
[*]You get married in the meantime, run a joint household with a shared account, and grow old together.

:-)

[*]Houses are always an inseparable part of the property on which they stand. The owner of the property is always also the owner of the house. The owner of the property is only the person registered as such in the land register (partial ownership is also possible, e.g. 50% father-in-law, 50% mother-in-law).

I have to interrupt here. I can’t confirm that. We have a house on leased land. The house belongs to us, the land does not. But I would also initially advise against building. For now. You can start saving and making plans. If you still want to build in 5 years, great. Then you’ll have a small equity and maybe already plans to get married, etc. Then it will read quite differently and the bank will also look much kinder. If the father-in-law wants to build, that’s his business for now. Don’t get lost in dreams that aren’t necessarily yours. A home loan binds you more than a marriage contract. If you’re still afraid of marriage, then the loan should definitely deter you even more.

I have to interject here. I can't confirm that. We have a house on leased land. The house belongs to us, the land does not.

Yes, of course, you are right. On leased land, it looks different. As a rule, at the end of the lease agreement, dismantling is necessary (for example, with garden sheds on leased gardens). AFAIK, there is no entitlement for the lessor to buy the house. Conversely, the lessor has the right to return the undeveloped land. In the end, you can take your house with you or tear it down, but not cash it in.

For what you have in mind, the property must be divided. You both have to register in the separated part in equal shares. For the loan, you both have to register as debtors. But woe if you separate... That will bring a lot of stress. If you should separate, you will hardly continue to live there, so it makes little sense.

Yes, is it buildable???