Derwent London has acquired the 7-year headlease interest in the South West Wing of Bush House for £13.5m before costs.
Derwent London already owns the freehold of the 103,700 sq ft office building.
Architects Stiff & Trevillion have been appointed on the refurbishment and extension of the building which could increase the lettable area to 130,000 sq ft with a possible start in 2022.
Paul Williams, Chief Executive of Derwent London, commented: “This acquisition accelerates our refurbishment ambitions for this exciting site as well as creating immediate marriage value for the Group. We intend to restore our part of this London landmark to the prominence it deserves, ensuring the most modern, adaptable and sustainable design-led offices.”