Southern Co. has asked power regulators in Georgia for permission to increase the budget for its share of two new reactors under construction at Plant Vogtle.

In a progress report filed with the Georgia Public Service Commission Thursday, the company asked for approval of a $381 million capital cost increase for the project. Southern's share is now expected to cost $4.8 billion, which is 8.6 percent more than the budget approved by the commission four years ago, according to the report.

"The increased construction costs include schedule changes associated with the time required to obtain NRC approval of the plant design, the translation of the certified design into approved construction drawings and the rate of production of certain structures comprising the nuclear island," the report read.

The filing also announced that completion of unit 3 is now scheduled for the fourth quarter of 2017, with unit 4 complete a year later. The original schedule had unit 3 coming on line in 2016. Southern owns 45.7 percent of the project.