to be safe, the neighboring neighbor wants an entry in the land register, for the building authority his written confirmation is sufficient (if I want to build in the rear area of the property), he had the same with his neighbor, everything is fine.
Humor doesn't hurt,
But humor won't help: you cannot change the character of an area by the neighbor's consent. Especially not in the way "if the residential building only stands where the commercial building used to be, then I personally consider it an existing building" (which does not legalize the change of use, by the way).
A commercial area is and remains a commercial area. That is spatial planning, not neighbor law.
If the building authority issues a permit there, you can derive a claim for official liability if the district administrator brings in the wrecking ball. But you do not gain legal protection for the supposedly permitted "illegal construction by grace of the town hall."
MayrCh and I are only trying to kindly point this out, and nothing else.