Architect Fletcher Priest is to submit a planning application for an overhaul of Land Securities' 15-storey Dashwood House in the City of London.
The developer has elected to refurbish rather than rebuild in a race to complete the project before 2008, when it expects the market for commercial space in London to drop away.
Land Securities plans to strip the building back to its structure, reclad it and add two floors. This will transform it from a 10,700 m2 building with 14 usable floors to a 14,600 m² block with 17 usable floors.
Mike Hussey, Land Securities' London portfolio managing director, said it was essential to complete the speculative scheme before the end of 2008 as he expected a slump in demand from occupiers in 2009 and beyond. He added: "We can make this a wonderful project."
Developer British Land is also aiming to complete a 23-storey tower at Ropemaker Place, designed by Arup, in the first quarter of 2009.
The speculative office market in the City is still uncertain, as vacancy levels are thought to be at 10%.
Minerva is thought to be considering dropping plans for a 50-storey City tower after it failed to find a pre-let tenant.