Initial questions about the topic of house construction

... the property is supposed to remain in the possession of the 'parents-in-law'?

If that is the case - it will also be difficult for you to get a loan.

So it is actually owned by the parents-in-law, if I see that correctly?

We have actually only been together for a year.. But the father really wants it that way.
And I would also like something of my own...

However, we have not discussed any details yet.

To clarify: The parents live on the property, which directly borders the barn.

Assuming we get married... and the house is already built. How does the situation then behave?

So it is actually owned by the parents-in-law, if I see that correctly?

We have actually only been together for one year.. But the father wants it that way.
And I would also like to have something of my own...

However, we have not talked about any details yet.

For accuracy: The parents live on the property, which is directly adjacent to the barn.

Assuming we get married... and the house is already built. How is the situation then?

My advice: Leave it as it is under the given conditions and continue to enjoy life.

converting a barn into a residential house...

First, the technical aspect: a barn in this project would only serve as a donor of exterior walls and, if lucky, also a usable roof, both of which would have to be energetically upgraded according to current standards. This significantly reduces the value of the gifted property.

Furthermore, barns are often built on property boundaries, which would require a building encumbrance registration with the neighbor for a residential house.

The whole thing is then supposed to be built on a kind of leasehold property, and it is unclear to me for what service (it cannot be the friend's contribution to construction costs) you want to be registered in the in-laws' land register.

You need very, very extensive advice – from a lawyer and a tax advisor.

Assuming we marry... and the house is already standing. How does the situation then behave?

In any case, not with less complexity; it remains delicate. Then a joint prenuptial gain would be brought into the marriage (refinement of the barn into a residential house, starting from the outfitting), as well as a (in-law) parental gift (barn up to shell construction, on foreign land). One of the few situations in which even businessmen understand how only three lawyers can come to seven opinions.

Marriage does not change the situation. However, your husband can then gift you a part of everything (costing "only" notary fees).

If you contribute to the loan after the marriage, you would be entitled to a part of the money after a separation. Unmarried, the money is most likely lost. However, the law is also evolving in this regard. It could be a tough matter if you do not regularly receive receipts signed by your husband.

The barn would most likely be demolished.