Located a couple of doors down from the RIBA's existing base at 66 Portland Place, the current Institute of Physics building will be completely renovated to create enough office space to bring all the architecture institute's London staff under one roof.
Construction is scheduled to begin in March 2014 and expected to complete by the end of the year, freeing up space in the existing premises for new exhibition and events spaces that will include a gallery of architecture designed by London studio Carmody Groarke.
Patrick Theis and Soraya Khan saw off competition from five other shortlisted firms to win the project.
"We look forward to delivering a high quality sustainable design that both meets the RIBA's aspirations for its new building and reflects the integrity of 66 Portland Place," they said. "We were intrigued by the potential synergies between the two buildings and look forward to developing these further with the RIBA."
"The selection panel was greatly impressed by all the shortlisted teams' initial thoughts, approach to the project and their experience and ability to deliver within a constrained timeframe," said RIBA president Stephen Hodder.
"Theis and Khan gave an exceptionally considered approach and clearly demonstrated how they aim to meet our aspirations. We were particularly inspired by the team's consideration of the relationship between our new premises and our main RIBA headquarters building, and how they had successfully delivered projects with such synergies in the past," he added.
The RIBA has taken a 43-year lease on 76 Portland Place.