This is a bit of a minefield. Firstly - you can take any photograph of any building or person if you are on public land. That means a road, highway, bridle path, footpath, track. You do not need permission though you may want to ask out of courtesy. I am a photographer for English Heritage and that is the rule that we never break.

One of my photo heroes is the late Fay Godwin. She produced a wonderful book called "Our Forbidden Land" in which she wrote about and illustrated the problems of access to areas of the countryside. She had a real go at landowners, Ministry of Defence etc. When introduced to the then chief executive of the National Trust, without ceremony, she pinned him to the wall and berated him over their policy of making photographers pay to take pictures of Stonehenge!

Personally, I believe it to be morally wrong to go on to private land to snap a private house, but I also believe it to be morally wrong to be prevented from freely photographing our ancient sites. I do not consider that Avebury and Stonehenge or any land under the stewardship of Nat Trust or EH is private land. It belongs to the nation and that means us. However in law I have no redress so I use a long lens from the highway.. A test case might be interesting if Nat Trust ever sought to control or benefit financially from a photo of Stonehenge taken from the road through the obscene razor wire. ;0)