Depending on planning, the scheme could become the Dutch international star's first building in the UK.
Koolhaas' firm, the Office for Metropolitan Architecture, has been working with property developer Stanhope since April, it was announced on Friday.
Stanhope project director Matthew Lusty told the AJ that Koolhaas had taken on the designs for the redevelopment of three buildings occupied by Rothschild's, located on St Swithin's Lane.
'We're looking at a number of different options for the update,' said Lusty. 'We've started having initial discussions with the City [Corporation of London] in terms of planning constraints and how we can usefully use the sites.'
Rothschild's has had a property interest in St Swithin's Lane since 1812, and has now decided to update its presence to meet the needs of a 21st-century expanding business, by consolidating into a single office building.
According to Lusty, the redevelopment that would include Koolhaas' work is just one of a number of options that the developer is considering, which also include a refurbishment programme.