“I have a 40′ pond on my neighbors land that crosses my property line by about 4 feet. I want to build a fence across my property line which would result in me building a retaining wall and filling in my part of the pond. The pond dries up during the summer months. Do I need to file some sort of easement with the property owner to be able to build a retaining wall even if the said wall would be solely within my property line?”
You probably don’t need an easement, if you are staying entirely on your side of the property line, but it would be wise to consult with your neighbor (and possibly an attorney) before starting work. First, to make sure that your neighbor doesn’t have any rights on your side of the property line (such as an easement, either explicit or by use). Second, because all neighbors have an obligation to provide “lateral and subjacent support”, meaning if you do something that causes a landslide on your neighbor’s property, you’ve got a problem. Proper engineering and advance information should help you avoid any problems in this area.