An additional 105 places will be created at Ringmer Primary School by 2015.
It was one of six schools approved for expansion by East Sussex County Council, allowing for more than 620 extra pupils. The others are in Eastbourne and Hastings.
“We are rising to the challenge of meeting the demand for additional primary school places and are looking at a range of options to meet that demand across the county,” said Cllr Nick Bennett.
“The pressure on primary school places is an issue we cannot ignore, with rising birth rates and housing developments creating a need for additional places. The expansions of these schools will help to ensure demand is met in these areas over the coming years.”