When you take a look at home additions in Atlanta, the most common type of 1st floor home addition is an addition toward the rear of the house. Usually this home addition is enlarging a Master Bedroom, expanding on the kitchen or increasing the living room. An expansion on the first floor also has the possibility of being less expensive then on a second or third floor.
As you consider that first floor home addition, also consider possibly finishing a basement. Finishing a basement might meet your needs and has the potential to cost less than expanding with a home addition.