I wonder whether you are trying to do it for the first time, or whether they are trying to stop any water running off. The short answer is that this could end up being a legal nightmare, so do everything possible to try and avoid legal bods getting involved. The legal term for this is an easement. An easement is the right of one landowner to restrict or constrain the land of another. More common easements are ones that relate to a right of access over private land, or a right to free passage of sewage pipes under another's land. But the same principle applies to water run off from one piece of land to another. What are you trying to do?