Should I allow a neighbor to build a fence on my rental property?

[Mark Malnar]

I own a single-family home that I use as a rental.  About 15 years ago, I signed a easement to let my neighbor extend their driveway about 3 to 6 inches into my property.  At that time I lived on the property and I didn't care.   now the property is a rental and it has been for some 13 years and the neighbor wants to now build a fence that would extend about 20 inches total onto my property.   I like the idea of having a fence but I'm concerned with if there are any liabilities for me having a fence put up by a neighbor on a rental property or future disagreements about who owns the property if they were to sell the house.  I've done many google searches, and have yet to find anything that satisfies my curiosity..  thank you in advance for the help.