Frost-resistant foundation of a wall next to an existing neighboring fence

Good morning everyone,

I hope the title is descriptive. It is about the following problem: our neighbor has built an L-shaped stone wall as a fence. Slightly set back from the property line (about 2cm).

He does not allow us to fill back against his L stones. Therefore, we plan to build a wall at the property line as well. This is on average about 70cm high, at the highest point 100cm.

How deep should the frost-free foundation be at this point? Also 80cm deep or can it be founded less deeply because of the neighbor’s wall?

If water can run under the lowest point, the wall can move, I think. And it can, even if something frost-proof is built right next to it. (my consideration as a non-expert)

How deep should the frost-free foundation be at this spot? Also 80cm deep or can it be founded less deep due to the neighboring wall?

Frost-free depth depends on your soil and climate. It is not clear to me how a wall 2m away should influence your frost-free depth.

it says something about 2 "Zentimeters"

Uh yeah. Got up early this morning :/ Basically, the foundation should be on the neighbor's side beyond the border. This would still require a frost-proof foundation on your side. Water could possibly seep over to the neighbor, but that is not certain.

If the L-blocks are only 2cm away from your boundary, I doubt that the foundation was completely on his property (but of course I cannot rule it out, it is possible). However, if he founded on your property, it shows a complete lack of common sense on his part to forbid you from backfilling (a demolition request from your side is possible if proportional?).