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Thanks for your answers. This is the fourth time he has started building, first alterations to house, second barn conversion next door, third, garage up against our boundary told us it would only be about 3 ft. above wall, it is about 10 ft. Built a kitchen on bigger than given planning permission for apparently, but he seems to get away with it!
I reckon the new gable end will be about 10-12 ft from our boundary looking straight into my garden and conservatory. I don't mind so much the extension, it's the two bedroom windows looking over us.