Building on the parents-in-law's property

: true... I have doubts, I think that's also why I'm asking here again, to reassure myself...

Sometimes I wonder if I'm seeing it too narrowly.

My partner would really like to do it, but by now I am really reluctant.

For us, there is still the option to buy land. The question is, if you're not married, is it better if one person buys it?
You can return it to the municipality within three years.

As someone who built next to their parents:

We had the opportunity, in a good location, to get a plot of land from my parents. With land prices at 270€/m², it was relatively clear to me quite quickly, as long as a few basic principles like privacy could be realized.
My wife was initially skeptical, precisely because of the proximity. Financially, however, it was so significant that we decided in favor of it.
My wife is also listed in the land register and on the financing.
We went through the whole thing with an architect and currently have nothing negative to report.

However, we are also lucky that my parents are very reserved about it, as they themselves went through it with my grandmother and had rather bad experiences. They want to do it better.

After now 2.5 years in the house, it has settled in such a way that we are very satisfied.

However, I would not advise you to do this, as it depends on too many details, depending on the plot and especially the relationship between you and the parents/in-laws.

With your land prices, even for me as a son, the decision would not have been so clear. Ultimately, you have to consider this for yourselves; however, I would neither discourage nor recommend it.

Fact is: Purely legally, this can be arranged with or without a marriage certificate so that no one has to take any risks - it may cost a little more without than with one. Whether you then want to continue living in your half house next to the ex-parents-in-law-to-be (worst case) is, of course, another question.

The person listed in the land register also owns the house.

The question is, if you are not married, is it better if one person buys?

It's a great thing. The partner buys and you are on the financing. So he: property and you: debt. It only makes sense if the one who buys also handles the financing alone.

Therefore, the way would probably be: subdivide the parcel, gift to son, half sale to LAG, joint construction.