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

he is completely lacking any common sense to forbid you from connecting

Common sense would tell me:
Building right on your own border cannot, of course, be forbidden by the neighbor, but burdening an existing structure on the neighbor’s property can be.

To me, a good relationship with the neighbor seems far more important than a good wall between you.
I would definitely take that into account when planning :)

I would assume based on common sense:
Building directly on one's own boundary cannot of course be prohibited by the neighbor, but placing a load on an existing structure on the neighbor's property can.

Far more important to me than a good wall to the neighbor seems a good relationship with them.
I would definitely take that into account when planning :)

I completely agree with your second sentence, but I do ask to be fully quoted if at all, because the quoted clause can also give a different impression (from me).

".... But if he has set up on your property ..." I had overlooked that. Presumably because I had understood it differently. If that were the case, there would be completely no grounds for this refusal, of course.

Good evening everyone,

thank you for the feedback and sorry for the late reply. I was busy all the time with the excavation and afterwards we were on vacation for a week. I have now excavated a trench over the 10 meters. On average currently about 70cm deep. Towards the end, however, only 20cm, because here I am, as suspected, hitting the foundation cushion of the neighbor's L stones (it also follows the slope and the L stones are 1m long). His comment is that I should simply "knock it away" (the concrete) and then lay a new foundation myself.

Actually, I had hoped that he would still be on vacation and I could simply fill up with the gravel that is coming tomorrow.

Perhaps the situation can be recognized in the attached pictures.

Whoever has a neighbor like that...

Build the wall as high as possible. But before that, he should remove his concrete wedge from your property.

Did he actually seriously put L-blocks into L-blocks? You could annoy him back with the question of whether that is approved by the manufacturer