New to Nashville (Davidson County)

[Elijah House]

I'm new to the Nashville area and am looking to buy a multi-family property in Davidson County. How difficult is it to get permitted for a multi-family when the property is zoned RS40 land use description "single family"? I'm looking specifically in the southwest neighborhoods (37209, 37205, 37215, 37204, 37220, 37212)? 

In this particular case there are other multi-family houses on the block with the same RS40 zoning, but the land use description is "duplex".

Thanks in advance!