@Sophia Berry "Buildable" means different things...
1st, take the name of the subdivision and the address of the nearest home to whoever has jurisdiction for Planning and Zoning. If you're unsure who this is, call the County Government. They should be able to tell you if it is them or some City, like maybe the City of Lithonia. You might call that City also to be sure they agree. Planning/Zoning can tell you what the lot is zoned for, for example they may say "R1" which probably means a SFR. Ask them if the Lot is "buildable" and if it is a flood plain.
2nd call the Building Department that has Jurisdiction. The PLanning/Zoning folks can tell you who that is. Ask them if the Lot is "buildable."
I suspect both of these will say "yes."
3rd Ask the seller of the lots for copies of the "Setbacks" and the "Covenants and Restrictions" for the neighborhood. The Setbacks are shown on a drawing and should show the boundaries of the lots and things like an "easements" which is an area you can build a house on - but you can probably put a driveway on. There may be an easement for drainage in a "swale" that you would not be allowed to build on. I would suspect that a drainage easement would be the likely risk in my area. Water will flow through that easement area during heavy rains.
The Seller and your realtor should be able to give you some sense of why those lots are leftover. It could be for financial reasons. Or maybe there is something about them, like these easements, that make them undesirable.
I suspect these lots are buildable as far as the parties above are concerned or they would not have been platted as homesites in a subdivision.
The 4th thing to check is sewer. If there is a sewer system in the subdivision it's likely that all is fine; however, if people there have septic systems then you'll need to check with whoever has jurisdiction for that, probably a Health Department in the County. Those folks will tell you the soil requirements. Normally there is a way to get a septic systems for most soils but some are much more expensive than others.
The 5th thing to check is driveway access permit at the County Highway Dept or City Street dept. I suspect getting a driveway permit will be easy or it would not have been platted as a subdivision.
AT that point you'll probably have an answer.
You could also as a Builder/General Contractor about considerations for building there. A Builder is the first stop for most people as the Builder typically pulls the building permit and takes care of all this stuff. Your main concern before buying is that the buildable space on the lot will allow you to fit the home you want, plus a driveway, septic system if needed, etc.
In my area an Architect would be optional and an Engineer (Professional licensed Engineer) would likely not have a role, his work already being done when the subdivision was originally platted.
A surveyor's role would typically only be to use the subdivision setbacks/easement drawings which are on file at the City or County to determine the actual physical lot boundaries - and those all important easements - and set out stakes or flags so that you can visually see where your boundaries area. He would be responsible for getting those in the right place.