Has the "structure" bit ever been defined? There is a lot of fences around.
The real problem is nothing to do with legality: if you are at 500ft it is virtually certain that somebody will report you for low flying, and they probably can read the number plate. You will probably get away with it (any half decent lawyer will make the point that a layman cannot possibly judge height accurately) but that's not the point. GA has loads of enemies already.