What can I do if my neighbour occupied a part of my land illegally?

My neighbour who lives next door re-built his house .Our house wall adjacent to his house was poorly built and left 2 to 5 inches of our authorised property land in the open which he cleverly occupied while building his house. Now his construction work is almost complete and he has said he did not know about it and would give it back when we construct our house (maybe in next 8-10 years) as demolishing entire wall on all the floors is not possible. There is high chance that they will shift their behaviour in future and would not give the occupied space back.Is there a legal way to ensure that the owner and his family stick by their words when we reconstruct our house and action taken against them if they refuse to do so(like a legal agreement which can be used in court).