casita encroachment onto another property

[Steven Mitchell]

We are about to close on a property.  Just got the survey back, the casita in the backyard encroaches on another property by a whopping 18 inches. The casita looks like it has been there at least two decades.

Is this an issue?

Just from some googling, it says if a property encroachment isnt immediately addressed, the person could lose their property due to the adverse possession law.  Or something like that.  

We are worried this will become an issue when we try to sell it. Any thoughts are much appreciated.  We are in Texas in a historic district.