Build new or wait for an affordable house to become available?

I would buy instead of building in your place:

1. financially, buying seems significantly more sensible according to the information you provided, also because you are young and a lot can still change
2. Passive house or not, but it doesn't seem to be your dream to build yourself. So why throw money away in a structurally weak area?

I also have no solution for the "waiting." How, where, and with whom you want to live, I refrain from commenting

I know all that, I wrote it myself in my post, that's why the

So if the infrastructure in terms of schools, etc. is available, plus the described proximity to Cologne and Bonn... then even the "double the costs" of 18€/sqm are such a bargain that I would love to buy everything immediately. Honestly, that's what I recommend to you too, before someone else gets wind of it. Regardless of whether the community prefers families... as soon as the first major investor waves the cash there, the principles are quickly forgotten.

The described proximity to Cologne and Bonn, yeah sure. So you consider 50 minutes to Bonn and 1:10 hours to Cologne as close? Then Landshut would be the Munich suburbs for you or what?

Well, this thread is getting a bit off track, I have contributed to it as well.

Let's summarize the facts:
- waiting for the nice old lady makes sense only financially, it does not fit the reality of life for a young couple with a small child.
- there are no other existing properties for sale in the village.
- happiness in life depends on living exactly in this village.
- the building land is practically given away, in addition the development costs will certainly apply. I would roughly estimate €40/m², still a gift.
- the mother is wealthy and guarantees the loan which otherwise would not be viable.

Conclusion: build and be happy.

2. Passive house or not, but it doesn't necessarily seem to be your dream to build yourself.

I think, in any case, a desire has to be sacrificed first: passive house OR tiled stove.

Building

Who knows how long you have to wait.

The used house can also be energetically renovated, but a new build requires less maintenance work. There is always something in a house from the 70s.

What about a practice and the associated office?
Definitely talk to the municipality. Here you get the red carpet rolled out.

Talk to the banks, so that in the end only the savings bank with high interest rates is left.

It may be that a new build is burning money. Losses are only losses when they are realized.

If I, purely as a mom, compare my quiet village with cities, I gladly burn money.