I'm 18 and want to turn a duplex into a triplex

[Noah Mount]

I am 18 years old and want to buy a duplex and add another unit. Is this possible/legal if I turn the garage into an efficiency room where the total area is about 500 square feet for the unit? (I live in northern Georgia if that helps.) I also have questions about how much people would think the contracting cost would be and how long it would take. I can send you pictures of the property via communication method. The price is $54,950 but I think I can get them down to $50,000. For a 5% downpayment, that would be 2,500 down, which I can save up in about 2 months. 3.5% down would be $1,750. I can pay for the property myself and have it be very lucrative if I do it right. (I've never bought a rental property before). I just need estimates from experienced investors so I can get as accurate of a budget as possible before buying the property. Thanks!