Westbourne Primary School in Sutton will welcome 30 extra pupils at the start of the next school year.
A single-storey extension will be added to the site in Anton Crescent.
The school has accommodated roughly 470 pupils since 2008, but is expanding to meet greater demand for places.
Three new maths classrooms will be built at Cheam High School in Chatsworth Road, Cheam.
Both developments were approved at a planning committee meeting on Wednesday, June 11.
Councillor Wendy Mathys, Children, Family and Education Committee chair, said: "Everyone recognises school places in the borough are at a premium.
"We will do everything we can to create extra capacity."