What happens if you have a problem and land/crash on their property?
Legally you need nothing to fly over someone's property because you are using the AIR and not their land but . . . you need to be mindful and respectful of their privacy and such. Even more so if you're doing this in an area close to home.
I always try to fly in the 200' range above anyone's property. They might hear it, but by the time they see it, it'll most likely be past their property. I tried watching my own bird flying into my yard, (and mine is an acre) and I usually locate it after I already flew past my property. I tested at what levels would bother me if one was flying over my yard. I expect at least 75' above my property as mine and would be offended if someone flew into "my airspace". I always try the the " if the shoe was on the other foot" reasoning when I'm flying. Not that its foolproof in everyone's eyes.