Problems with neighbors due to slipped soil

I don't think the thing with the stairs will work. Because he has 3m driveway width from the house wall/door to our property boundary. To overcome 1m you need about 3 steps? That means forward it probably also takes up about 80 - 90 cm? That means about 2.20m driveway width remains.

1m are rather 5 or even 6 steps.

You can also make a small platform and then have the steps not away from the house, but next to it, that works and also fits a car, it’s like that at my in-laws (so a C-Class and my Passat can pass by there).

If he can build his garage 1m deeper, that's fine with us of course. Then I'll just put the earth back there and he has to dig it up again. That problem would be solved for us for now. I'm looking forward to the conversation on Thursday. Apparently, he wants to bring his building permit for the garage next week, which he currently does not have. I will report then.

We had the conversation with the neighbor on site today. As expected, he naturally did not have a building permit for the garage. He needs to lay the foundation about 1m deeper. He then went to the building authority and called us afterwards. He now wants to apply for a subsequent terrain modification and for that we and the other two neighboring property owners (currently the properties are only reserved) must agree or approve this. I told him that I need to reconsider this, as I want to discuss the consequences for us with our architect. In addition, I am concerned whether I need to register a building encumbrance for this, since his garage would have more than a 3m height difference from the original ground level. On the other hand, one also does not want an angry neighbor who could not build his garage because of us. How would you proceed?

This is the best approach for you. Talk to the architect so that YOU have no disadvantages. For me, it is a disgrace that your neighbor claims to be the "big shot" and then the facts turn out differently. Fascinating topic!

Have you talked to the other neighbor about what he thinks of the dumping?