Neighbor is building a garage deeper than the development plan specifies

The supposedly incorrect height is a matter between the neighbor and the building authority, but the retention of the terrain is between the neighbor and the OP. Unfortunate. Perhaps the building authority can be dragged in front of the proverbial cart in the hope that the neighbor will also add soil .... realistic?

In fact, the OP made the mistake here of adding soil without securing it. Even for an edge, he will have to dig up his stuff again, especially for L-shaped stones.

A different question. When I read [Fertigfußboden] then this is the specification for the house. Is this height actually mandatory for the garage, parking space, carport?

Is this height actually mandatory for garage, parking space, carport?

Yes, the exact stipulations from the development plan are missing here. And if the neighbor had not built any shelter for the vehicle at all, the embankment would have had to be retained as well. This could only have been avoided if one had coordinated with the neighbor beforehand.

Yes, normally yes, but it can't be that one person follows the development plan and the other doesn't, and then you also have to pay because you filled it up?

What does the neighbor say? Here in the forum, one can only speculate whether and how this regulation was circumvented. Ring the bell, bring beer, discuss?

Since the height difference is less than 1m, there should be no need for a protective railing or a wall.

The regulation refers to the ground floor; small garages usually have no floors, or they are buildings again, whereby afterwards a deviation from the development plan would rather be possible for the neighbor.
A garage can also be an underground garage or a lift garage.
What speaks against it in the development plan, what was approved?

Maybe it's just a lot of fuss about nothing and an architect was possibly smarter?