The contractor platform extensions are needed to prepare for the east and south London routes for longer trains that are being introduced to help meet a predicted rise in demand of 40% by 2021.
“We are pleased to be working with Cleshar on this crucial project and these upgrade works will be completed with the absolute minimum of disruption to services,” said TfL's director of rail Jonathan Fox.
The work is part of an upgrade programme to deliver a 25% capacity increase to meet rapidly increasing demand. London Overground trains are being increased from four to five carriages.
The first of these trains will be running by December 2014. Works to extend platforms will get under way in autumn at stations on East London route stations at Highbury & Islington, Canonbury, Hoxton, Haggerston, Dalston Junction, Shoreditch, Shadwell, Wapping, Canada Water and Surrey Quays on the east London route, as well as Wandsworth High Street, Clapham High Street and Clapham Junction on the south London route.
Fox said: 'Although not yet six years old, London Overground must continue to grow its capacity. It's hugely popular, with passenger numbers increasing by 87 million in its first four years of existence. While that rate will slow, we must still be ready to provide for the 810,000 new residents who will come to London by 2021.
The work is part of the £320m London Overground capacity improvement programme.