[Highland Park] how to resolve property line disputes?

Hi, When we bought our house two years ago, we had an official survey made that shows our property extending about six feet beyond our back yard garage. We recently discovered a neighbor was doing construction in this space, and he claims he has a survey showing it is his. We tried talking to him to no avail. Does anyone have experience resolving property line disputes? I'm guessing I will need a lawyer's involvement. Assuming so, does anyone have any recommendations? Thanks so much, Sue