Duplex in Atlanta with possible inaccurate public record

[Ant Hony]

I am looking at a duplex investment in Atlanta and plans to use the BRRRR strategy.

I am not too sure how the appraiser will value the property since it is legally zoned as a duplex and it has two meters. However, the 2nd unit is "below-grade" and the square footage on tax record does not reflect it.  The property was built into the side of the hill where the lower level is significantly out of the ground. Also, the number of rooms in the record is wrong as well. 

How does appraiser approach this type of home? I understand that most typically do not consider below grade area as GLA. But, would they consider it in this case since it is legally a duplex?