Duplex Definition Help?

[Zachary Kane]

Hello,

I am looking to buy a rural house in South Carolina.  It is a large old farmhouse that has lots of additions to it over the years.  On the second floor there is a back staircase and a sort of "in-law" suite with a small kitchen that connects to the rest of the 2nd floor bedrooms.  There are two full baths on the second floor. 

The appraiser is saying that the house is a duplex because of this situation.   I guess in theory somebody could live in that suite if the doors to the rest of the 2nd floor were locked, but why would the appraiser be pushing this approach when it is not listed as a duplex on the listing or on the county tax roles and nobody plans on using the house as a duplex?

Is there a location where I can see what the true legal definition of a single family house is vs a duplex?

Thanks for any help.

Zac. Kane