Turning a SFH into a legal Duplex

Hi Everyone!

I'm seeking advice on how difficult it may be to convert a SFH with an ADU (accessory dwelling unit) into a legal Duplex. I'm sure this process varies by state, city etc. but figured I would take a crack at the forums to see what I could learn.

I've found a SFH in the Myrtle Beach, SC area that has a "mother-in law" suite that has been built with a separate entrance, separate electric meter etc. From what I'm being told, this addition has been built to code and although this will be confirmed once the inspection is complete, I am trying to better understand the process to convert this property into a legal duplex. I want to ensure that I would be allowed to rent out both the SFH & the "mother-in law" suite prior to purchasing the home. I've been told by the county that the property is zoned for a 2 family, so we are in good shape there, but it will need to be reviewed by the "code" department and would need to be legally converted from an ADU to a duplex in order to be able to rent out both units.

Has anyone gone through this process before and may be able to lend some advice on how long a process like this can take? Is it typically expensive? What all does a process like this entail?

Any insight at all that is provided would be much appreciated - thanks in advance!