When is it considered boundary construction?

Doesn't it still stand on the boundary then, or is it that narrow at your place?

Yes, of course it stands on the boundary then. But along the entire length, the property boundary is practically on the neighbor's land (about 20cm). My idea was whether that would count differently or not. After all, it is built over the boundary. If not, then not...

And then, primarily, it was also about the neighbor— is it enough if he agrees and I have that documented in writing by a notary? Or should it rather be registered as a building encumbrance? If the neighbor agrees to the building encumbrance, can it then simply be registered? Or does the building authority or whoever have the final say and can still say no?

Perhaps as a small hint. At least for us, there is a maximum length per border (9m) and a total length of all border constructions (I believe 15m). Maybe that already solves your problem.

That is known to me.