Can anyone point to the actual legal requirement?
This was something much discussed some years ago in the crew room when we ran out of politics and religious subjects to discuss and the conclusion that we came to was at least a civil offence of trespass (depending on where and the the circumstances). It can all get very grey, as with most law.
We then sought further legal advice and the following was given:
Permission may be implied (as in welcome sign, public entrance to public parking etc)
Permission might be conditional (footpath to letter box, closed gate at end of drive)
Permission may be by stautory right. (Public footpath, neighbour right of way, water board electicity company, warrant, court order etc)
If you have none of the above and no owners permission then you are probably trespassing (with another million bits of grey area)
Some food for thought though if you get it wrong. The owner will have recourse for your unlawful act and can consider a landing fee or lien and padlock your machine to the ground until you pay (Its been done!).
I much prefer the ask nicely, or apologise profusely method. A friend of mine keeps a bottle of champagne in the back, along with a nice smile just in case he ever needs it for this very reason.
The other method of course is to get Flying Lawyer and his colleagues to help you out which in turn helps him with his own landing fees. (Only joking! I am too old to sue and I fly therefore I have no money left)
Sometimes its better not to ask the question
AOG