Apt Above Garage has "Storage only" permits, How to make Liveable

[Allan Smith]

Has anyone dealt with this before? I'm looking at purchasing a house with a separate building that has garage and 1000 sf apt on top. The permits pulled say it is "storage only" but there's a bathroom and kitchen. It's R10 (In Nashville that means two units is allowed as long as lot is 10,000 SF which it is) so a second unit should be fine, but if I have codes come out to make the apt officially legally livable, what would happen? Would they be able to simply update after seeing it, or require me to tear out drywall, redo stuff, etc? This would allow me to sell the property with apt as official living square footage.