Who has to support the property?

Then just remember that, if applicable, retaining walls also require a building permit.

I think (hope) that he won’t oppose this. He needs about 10m of retaining wall at the back to raise his garden so that he can use it – I need 10m of retaining wall at the front to build my garage. Then there will still be a few meters left that can also be built on to create a continuous wall. If we split the costs 50:50, each of us will probably have to pay around €10,000.

If he doesn’t cooperate, he will never be able to use his garden door again – it currently hovers about 0.5-1m in the air because his garden isn’t raised enough. Beyond that, his garden is very uneven because he only built up one corner of the garden – without a retaining wall, he can’t do anything more there.

If he’s not willing to have any discussion at all, I’ll just build the retaining wall on my property, as high as I need it (and as far as allowed), and he can figure out what to do then.

Well, with that attitude I’d give you a piece of my mind^^

Well, with that attitude, I'd give you a hard time^^

Just setting the neighborhood relations straight right away. This will make living side by side wonderful,...
I would suggest a more conciliatory approach to you, as HilfeHilfe already points out.

You have surely misunderstood me... Of course, I will first introduce myself politely and try to find a way together that suits both parties.

If my neighbor totally refuses and under no circumstances wants to settle anything with me (because that has been the impression I got from him so far, but I have only exchanged 2 sentences with him), then I will build the walls there as I need them on my property. Of course, I will not come in with a demanding attitude right away, that's clear; first I will ask how he imagined all of this.

Isn't it the case that the person whose property is higher up has to ensure that neither soil, mud, nor water flows onto the lower-lying property? If you place yourself higher, you must secure your property. Or am I mistaken? I only know it as the person with the higher property paying, but often one agrees to share the costs.

In our case, there was already a wall. It no longer looked trustworthy and was not only very old but also extremely ugly. I admit, I have a quirk in certain things. I like it neat and appropriate. :rolleyes _O Our neighbor thinks similarly in this regard. We form a mini street with three houses in the street, and we would like everything to create a nice picture together. He is currently adapting his driveway to ours. Advantage: he can get into his garage much better, and the driveway will no longer be as steep as before. Also, when backing out, he can turn into our yard and no longer has to reverse down the driveway. We often brood together on how to do it well and inexpensively. But: the chemistry was right from the start - it was "love" at first sight. I would invite your neighbor for a coffee or a beer. Discuss everything with him calmly. I can imagine he might be interested. Maybe he can even use part of your excavation. Maybe he is just introverted or shy.