Defining the financial framework, recommendations on house sale / land selection / new construction

A house serves quality of life. If you can significantly improve your quality of life with another house without overextending yourself, do it. Whether it is a different commute or any other aspect, it does not matter. The only important thing is that the project is not approached from a "I want to get away from here" feeling. In that case, it would first be necessary to examine what problems are taken along when moving (usually at least yourself). Our experience with new construction at the end of our 40s and teenage children has been exceptionally positive.

Currently, we live on 2 full floors, with 3 bedrooms and a separate office, and 150 sqm. We can imagine downsizing to a bungalow with 2 bedrooms and no separate office. So preferably something around 100-120 sqm.

I think it's a great idea. The right house for the life situation. I haven't done it any other way since my mid-twenties. That also answers the budget question well.

Basically, you can and should do whatever you want.

The house that fits the life situation.

I currently don't see any other life situation here. The child is currently 15, will move in at 16/17.

that’s why we came up with the idea not to build a two-story city villa this time, but a house that we can also live in and manage without kids and all.

In a house of about 100-120sqm living space. I bet with you: when it comes to the planning, it will end up being 140/150sqm again. The child needs space, will live with you for quite a while longer, you can’t plan that away or reduce it. Or rather, I can’t imagine that you do that.
For me, you are 10 years too early. I think the desire to change a house may have other reasons. I remember a thread with a plot near a nursing home...
But an actual need for change becomes interesting when you have health or physical changes due to age. You can consider some things now, but the “wish list” at 60 is truly different than at 45. Few admit it, but unless you feel like you’ve been run over by a tractor at 45 or 55, or you get the chance to change, you don’t think about this or that.
But while I write this, I think you could probably build a current kid’s room so that it can later serve as a second bedroom. However, there will probably still be a need for an office since about 20 more years of work play a role.
About the residential area: nice surroundings. Personally, I would pay attention to the mill view and would not want to live between two two-story houses if the plots are as big as they are. The prices are crazy though!

The child needs space, still lives with you for a while, that can't be scheduled away

For a house that is almost planned for retirement, the youth room is what it is called today and a guest room tomorrow. Then 100-120 sqm are completely sufficient.

The child needs space,

10 sqm. Done. Then it's 110 sqm for the parents.

I bet with you:

That sounds a bit know-it-all. If both are clear about their plans, that's fine.

But while I write this, I think that a current children's room can possibly be built in such a way that it can later serve as the second bedroom. However, there will probably still be a need for an office, since another 20 years of work will play a role.

A room can serve multiple functions. Both simultaneously and alternately over time.

A room can serve multiple functions.

I probably don’t need to be told that. I preach it.

That sounds a bit know-it-all.

Funny. Lately, it’s you. You can’t put everything in one drawer.

10sqm. Check. Then it’s 110sqm for the parents.

I guess it’s not in their interest to reduce the child’s room (or teenager’s room, no matter what you call it) when the child is a teenager.

If the two are clear about their plans, that’s fine.

So clear that they’re asking here. I think the forum is a way to collect several opinions. Maybe in your renovated house you don’t have more than 10sqm available for your children. Maybe you would think differently if you could provide 20sqm for each of your children. I find your objectivity lacking a bit. It doesn’t all have to be like you. Be that as it may: asked, and the question is addressed to everyone. And there’s nothing know-it-all about having an opinion that refers to previous communications with the OP (even if the term “bet” is used). By the way, such blanket answers

Do it! ..

are also unnecessary. A bit of empathetic reasoning wouldn’t be wrong either.

I probably wouldn't do it. The rate is about 900 euros higher than the old rate. Your husband will retire during the repayment period, maybe you will too. Can you still easily manage that then? Building a house is always stressful and time-consuming. You will be occupied with it for at least 1 year. Even if everything went well the first time, it doesn't have to be the case the second time. And even if everything goes well, it is time-intensive. At least 1 year, possibly longer including moving and setting up again, during which you do nothing else (except work). Then you have to lay out the garden. Before it looks nice, another 2 to 3 years will pass. You save 20 km commuting distance. That should not be underestimated. But how often do you drive it? You have an office, so presumably you don’t have to drive daily. For me, another question would be: what is the location and size of your current plot? I don't particularly like any of the new plots available, I would probably choose 22 or 25 if I had to choose.