Distance from terrace to property boundary

One can also, as a neighbor, request access to the building file at the building authority.
One must prove a legitimate interest for this. This would generally be affirmed if one could be harmed in their neighborhood rights. The most common case for this is current building procedures.

One could therefore request access in writing by stating the legitimate interest.
Alternatively, one could also obtain permission from the neighbor to inspect it.
According to the statements of the OP, the neighbor claims not to have changed the original topography and therefore should not fear the file inspection.
At least, one could argue it that way.