To note to the OP that parallel parking spaces have the typical 2.4m spacing requirement (less if 30 degrees angle parking) to allow for doors to swing open partly over the adjoining space. And there are strict requirements about objects not blocking those door swing areas.
Not so for storage space areas.
So you could “store” eg a 1.5m high barrier or other tall objects right along your marked border line.
That might not make your neighbour too happy. And it might not help you either! But it might be a point for negotiation if the neighbour insists on the “it’s not a car space” argument.