Georgia Power and its partners are trying to build two nuclear reactors at Plant Vogtle near Augusta.
Early site preparation started in 2009.
But the company can't get the federal license to build the units because manufacturer Westinghouse keeps re-designing them.
Nuclear engineer William Jacobs told members of Georgia’s Public Service Commission design revisions have caused the company to change construction plans.
That, he says, could boost costs well over the project's approved $6 billion budget.
"Those costs that I’ve identified they could be large they haven’t been quantified yet and also the responsibility for those costs has not been determined."
The costs would likely be spread between Georgia Power and other members of the consortium financing the project. Ultimately they could be passed on to ratepayers.
Georgia Power says it is working to manage costs and doesn’t expect any problems.