20 years ago we changed our driveway from a rear entry to a side entry and had a retaining ewall built with railroad ties. It extended onto the neighbors property by about 3 inches at the very bottom of the drive. We had written permission from him at the time. Now the wall is rotting and needs to be replaced. Our neighbors who live there now are happy to have the eyesore fixed. We want to have a stone wall built and replace the ties. However, the stones are wider than the ties so it would extend onto their property a few more inches. They are happy to sign something that gives us permission to do that. Are there any legal issues we need to think about before we go ahead with this?