Garden box base plate directly on the boundary or better some distance?

that in a boundary construction you really have to build directly on the boundary and no 30cm or whatever distance is allowed

BW: Either on the boundary or at least 50cm distance. I would choose the latter.

I would also not place the garden box directly on the boundary. You might need to go around it once for painting (if wood). I would rather design the box as a shed / garden house with an overhanging roof. Then you can also place things against the outer walls all around (storage area). For example, stacked wood, beer garden furniture, etc. So about 1m between the boundary and the overhanging roof/edge of the base plate.