Probably not. Generally speaking, unless there is an easement or other grant of rights, a property owner controls not just the ground level access to their property, but access above the ground, at least up to a reasonble height. If your A/C unit is trespassing over their property line, the fact that it is off the ground does not make it any more legal than if it was on the ground--otherwise, that would be an easy way for anyone to get around another parties property rights. If your neighbors complain, you may be forced to move it. (As an alternative, if they complain, you might be able to negotiate with them--pay them something for the right to extend over their property, or grant them some right over part of your property that helps them. If you do, just be sure to get everything in writing.)