For what reason? Trespassing, common courtesy, local regulation, national regulation, privacy reasons, etc.? This is a hugely loaded question as it is. Might as well narrow down what is going to be a non-answer anyway.
In the US you can't interfere with someone's reasonable right to use their property as it was intended. If you are 1" in the air you are not trespassing. When it comes to local judges and right of privacy you might as well toss all the laws out the window as it's going to be a roll of the dice each and every time. In the end a good rule of thumb is if you think someone might take offense.... don't do it.