The London-based architects were to renovate RHS Lindley Library in Westminster but that plan is on hold.
The RHS has appointed Christopher Bradley-Hole to landscape Wisley’s £50m new developments. He will start on site in 2016, with the entrance the first project, followed by landscaping round a proposed science building.
The RHS said: "To the right of the iconic Laboratory, we’ll create a larger garden entrance to cater for our 1m plus visitors each year. The new structure will incorporate an exhibition area, catering facilities and a bigger retail space.
"To better meet the needs of these visitors, we will provide an arrival experience befitting a world-class visitor destination. The improved visitor entrance will include an exhibition space. Plant sales will be at the fore, to better engage with visitors and encourage more people to purchase and grow plants.
"A dedicated space for specialist UK nurseries will promote plants and offer expertise through a series of events. This will not only support British horticulture, but also gives more people access to an even wider range of gardening knowledge and plants. The Wisley shop will stock our unrivalled range of garden sundries, horticultural books and gifts, alongside new café and restaurant facilities, and a prominent plant centre.
"The much-loved Arts and Crafts-style Laboratory building will be revealed in a setting suitable for its heritage status, and the surrounding landscaping and car parks will be in keeping with the entrance to our flagship garden."