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Thanks sp1214 that helps somewhat. He only told us last Sunday not a month ago. My husband said ok.
It does say if agreed the wall should be built half on each side. If not agreed he should build it on his own side.
He has just started the foundations and has quite a lot of large boulders he is obviously going to use to build a dry stone wall. They are quite thick and he has encrouched on our side with the foundations. But if agreed it says half each side.