Building on the parents-in-law's property

I am very torn because I think you would never really have privacy there.

Listen to your gut feeling. Without having read the other comments... breakup... interference... etc. Imagine your partner dies unexpectedly and you continue living there. What about a new beginning? That's simply not possible. The parents would see that as a betrayal of their son.

We could have built on the property both at my parents' and at my parents-in-law's (with prior division, of course). But fortunately, we agreed that we did not want either. (Although we have a good relationship with our parents/parents-in-law.) However, it is really a difficult decision, as it is also financially lucrative. It is best if you agree on this decision. Of course, there are advantages and everything can go well and everyone can live happily.....

Without wanting to offend you, I do think that it's a bit of comparing apples and oranges.
Building next to the parents is one thing, living under the same roof with them is another.

Apartment and single-family house

I didn't mean to say that the situation is 1:1 comparable, but at least similar.

For us, it's not directly living in the house that's the problem, but rather the shared outdoor areas. And to me, it reads more like the outdoor spaces would be used jointly. Therefore, I would advise against it based on my personal experience.

If everything were completely divided (own driveway/yard/garden), it could work (if the parents-in-law don't interfere too much).

But it really depends on the individual situation.

For your information to everyone, we have buried the topic and are currently working on a municipal building plot. Somehow, I have been able to live better with it.

... I am very torn because I think that you would never really have privacy there. ...

I assume this remark refers to the in-laws' property or also to the other properties? I will simply assume the former: Ultimately, in my assessment, it depends on how family- or origin-family-oriented both of you are. For myself, I can say that such closeness to parents or in-laws would never and under no circumstances be an option. However, there are commentators here in the forum who live like that and apparently have no problems with it. That you have been considering it for 3 years already shows me that both of you have doubts.

That rather speaks against an additional land purchase if the parents-in-law provide the land.

From my point of view, this is where the problems already begin. The forum member and her husband are living beyond their means, as they cannot afford the house and the land, and are thus ("forever"?) obliged to gratitude and accommodation. In my opinion, such an imbalance also damages otherwise fairly good relationships between children and parents. And sons- and daughters-in-law are a whole different matter.