The masterplan for the revamp was approved by Westminster City Council on Thursday, after the concerns of English Heritage (EH) and the council regarding the renovation plans were allayed.
The architect behind the scheme, Neil Bennett, managed to convince both bodies of the merits of creating an arched opening on the ground floor close to one of the RI's principal staircases - the Grand Stair.
A second cause for concern was a potential extension to what RI director Baroness Susan Greenfield calls 'the sad stair' - a dilapidated stairwell to the rear of the building.
Bennett told the AJ: 'We had discussions about the two interventions into the listed building with EH and the council, who ultimately decided upon the officers' recommendations that what we had proposed was acceptable.
'Our proposals provide access to the heritage of the building, as well as physically housing items concerning the RI's history as you make your way around it.
'We're essentially replicating the circulation space extant on the building's first floor on its ground floor and basement,' he added.
Pictured are shots of a stepped glass atrium in the centre of the RI, which will provide access to different floors via a central glass lift.
Subject to detailed planning and the results of a bid to the Heritage Lottery Fund, it is hoped the £14 million first phase of the masterplan will be under way by the end of the year.