As high as you can is best.  If the neighbors are going to be upset, you 
might as well have a good signal while they do it.  You get what you can, 
but 20 feet will work if necessary.  If it is just for recieve, height isn't 
as critical, but people still don't like to be clothes lined by a wire.
You do what you can with what you have, and enjoy ham radio.  Height makes 
much more of a difference on VHf and UHF.
You are never going to live in the ideal plane for radio, so just enjoy.