We've lived in our house for 20 years with a vacant lot next door. There has been a fence on what we thought was the boundary since we bought the house. New owner of the land next door came in and provided a survey which claims land inside our fence. I would think that Adverse Possession would make this a clear-cut case, but he is claiming two things: 1. Since nobody was living on the land, there was nobody to know that there was encroachment, and 2. Since we haven't had a lot line adjustment, the line established by the fence doesn't count.