PS with the caveat that I am not a lawyer, I would be sceptical about the argument that a by-law giving exclusive use of some part of the common property (for certain limited purposes, perhaps?) gives exclusive use of air space infinitely above. If permission is for exclusive use of a balcony that is part of common property, it is permission to use the common property that is described as that particular balcony, not the common property that is far above that. An example might be when a person in the flat immediately above has a separate permission to use a balcony which is a different part of the common property but directly above the balcony below. In this case the permission is to use a garden area, not somewhere far above the garden. Common sense says they have permission to use sufficient airspace above the garden to allow them to walk about and enjoy the garden in the usual way, rather than having to crawl but unless they are so tall that they would bump their heads on a balcony I don’t think they have an argument.