Building an Extension

We are thinking of purchasing a detached bungalow, but I have noticed that the house next door have built an extension on to their kitchen that goes right up to the boundary line that divides the two properties. If I put a fence up along the boundary (which is clearly visible), it would mean that I would have to step around this extension by about 12 inches to miss the extension. Or use the extension as part of the fence. My question is:- Surely this cannot be right can it? The extension has a window size of appx 4ft x 6ft clear glass facing a wall of about 1 metre away which is the bungalow that we want to buy. Please help.