................................................The only really safe option is a green zone around Avebury where no further building (with the possible exception of small improvements and extensions) is allowed..........................................................................

But the Garage already has planning permission as it has a building on the site already, it just needs change of use. The 5 houses that will be built will be looked at as an improvement which would be passed without a problem as the garage was becoming a mess. They would have looked at any impact on the protected area which is already in place around the stones. I know this is harsh but that's the law....
I'd love to buy one of the houses......

As for the land behind, it may be the case that planning permission is not available on this piece of land. If they were to build on this land there would be masses of red tape to go though first and the people in the surrounding area would be consulted that includes the national trust.