The scheme, which was given the go-ahead earlier this month, will provide improved office space for the Corporation, as well as creating a new public piazza.
The North Block, designed by Giles Gilbert Scott, will undergo the most significant re-development, with an extended floor plate and a new double-skin glazed facade and top floor.
There are also plans to create a new landscape, featuring plants, seating, lighting and a café, to transform the area 'from a windswept podium into a new public open space for the City'.
Meanwhile, initial refurbishment work on the complex's West Wing and Justice Rooms is already under way and is due to complete later this year.
Architect Yvette Hanson, of TP Bennett, said: 'We are delighted to have achieved planning permission, as the Guildhall is such an important landmark in London. It is steeped in history, having been the centre of City government since the Middle Ages.
'The challenge is to provide the Corporation with a modern working environment that also responds to the traditions and working culture of the Corporation.'
She added: 'It is a great opportunity to improve the quality of space within these buildings, their relationship to each other and the public realm.'
Work on the North Block is due to commence in Spring 2006 and is expected to finish in early 2008.