Can you explain a bit more? I thought the walkway was on your land, between your garage and his house. Now you say that, in effect, he has built a little extension between your two houses, filling in the gap. This is then a different picture because where does the walkway go, is it /was it to his garden or yours? If it's his and on his land that probably he can do what he likes, but he should still not attach his roof to yours without getting your permission, or do anything to the wall of your garage with might damage your property. Can you describe this a bit more clearly, please?