Legal Question in Real Estate Law in California

You could do a "lot line adjustment" which would move the boundary shared by the two lots. This requires hiring a surveyor, and applying to the local land use authority for approval, etc. An easier way, perhaps, is for you to leave the lot line where it is, but to grant an easement to the neighbor to occupy your land. You don't say what exactly encroaches on your land. If it is a permanent structure, you're not really supposed to use the easement process, but some people do it. You need to consult a real estate lawyer in your area.

Mr. Collins is pretty much right on. Your local planning and building department (or planning if they are separate) would have jurisdiction over a lot line adjustment such as you are proposing. That is who I would approach first and they will let you know what paperwork you need to submit. Mr. Collins mentions a survey, which will indeed be required. What other paperwork you need will depend on your local jurisdiction. It is NOT like a full property sale at all. You will need an agreement between you and the neighbor as to how much you will be paid, and who will pay for the survey and other fees and expenses. It doesn't need to be a very long or involved agreement, but should be drafted to cover everything that the local agency requires, who will do it and who will pay for it.