Gifford Wood in Upshire, Essex, will have 4,500 new trees, many of which will be native to southern Europe, as an early defence against global warming.
The City of London Corporation, which owns Epping Forest, plans to plant a mix of native oak and hornbeam alongside alder, birch, beech, cherry, field maple, rowan, small lime, and wild apple from local suppliers, and create glades and ponds.
It paid £270,000 for the 12ha of land and is fundraising £110,000 for the trees. Epping Forest superintendent Paul Thompson said he wants the community to get involved. "People can come back and say 'I helped buy these trees'," he added.
Part of the reasoning behind the "fantastic extension" is to stop housing creep and extend wildlife habitats.
Gifford Wood, this year's Lord Mayor's Appeal, is named after Lord Mayor of the City of London and corporation head Roger Gifford.