The popular venue opened in 2005 as a sensory space but parts of the site are now worn out or in need of improvement, the proposals said.
The project will see the cafe, store, covered seating area and Curiosity Corner demolished, with a new cafe block to be built, while the atrium and greenhouse will be kept.
Rotherham Council’s rule for expansions within the greenbelt is that the volume of the buildings can increase by a maximum of ten per cent.
Winthrop Gardens’ new set-up will be 7.6 per cent larger – and RMBC has attached a condition to the permission which ensures no further buildings can be erected without the authority’s approval.
The cafe will be partly built into the landscape to help minimise the impact on the greenbelt, while the roof will have solar panels and rainwater will be harvested.
There were no objections to the proposals but 36 letters of support, which praised Winthrop as a much needed and cherished community base.
Wickersley Parish Council, which has run the Second Lane site since 2016, said the new Winthrop Gardens would be “safer, healthier and more enjoyable” for visitors and volunteers.