we own a house which is semi

 we own a house which is semi detached to a smaller house. This is owned by someone who had only an interest apparently a responsibility for his part of the garden which he has since discontinued totally. He just cut everything down weeds garden plants- roses and pruned the few grapes too late. The hous itself is abandoned and has broken windows and we have mended the roof several times as it was letting damp into our house. He has been gone about four years now, and the last we saw of him he just looked and went ( away with his new wife). We would like to buy rent or squat his section, but are afraid of kindling and interest in irt for him. With us next door being a busy family with lots of animals he will never be able to sell it independently. What is our situation regarding this and how much would we have to pay to buy or rent it (and restore it )?