Where I live, there are bits of common property that nobody but the owners of the adjacent unit is ever likely to use or want to use. We have a written policy from decades ago that unit owners are encouraged to look after the plants in these areas and blend the planting and landscaping of these areas into their own area so as to avoid the appearance of hard boundaries. Unit owners are not permitted to erect a fence that would prevent access to these area. Fences are not encouraged but they are permitted. If you want a fence it must be not extend beyond the actual unit area boundary. This arrangement has worked pretty well.