An outline planning application for the scheme, which will transform the shopping mall and create 1700 jobs, has now been lodged with Glasgow City Council.
The project is to create 65,000sq m of additional shopping, restaurant and leisure space, including more than 15 new restaurants and a multiplex cinema.
The existing shopping centre will undergo an extensive refurbishment providing visitors with a first-class destination both day and night.
The project will link into Queen Street Station and there will be a new 1700 space multi-storey car park.
It was originally planned to build the car park above Buchanan Bus Station, but the move was opposed by Strathclyde Partnership for Transport and bus firms warned the work would cause traffic chaos for up to two years.
Instead the new car park will be built on land at Queen Street station.
Owners Buchanan Partnership also plan new frontages for the shops in Buchanan Street, Cathedral Street and Killermont Street.
They say that will improve the experience for visitors and support the regeneration of the Buchanan Quarter to create a more vibrant city centre.
Buchanan Partnership is a joint venture between Land Securities and Hender-son Global Investors.
Company bosses say the extension to Buchanan Galleries will complement Land Securities £70m shopp-ing and residential develop-ment which is now being built opposite the shopping mall, and the extension and refurbishment of the Royal Concert Hall.
It is hoped work on the shopping mall extension will start next year and take three years to complete.
Nick Davis, development director retail at Land Secur-ities, said: "Our aspiration is to further strengthen Glasg-ow's position as a retail destination through the creation of a new and integr-ated retail and leisure heart within the city centre supp-orted by world class public transport infrastructure.
"The development offers the opportunity to attract new international brands to Glasgow to complement the existing retail offer and maintain the vitality and vibrancy of the city centre."
Chris Pyne, director of shopping centres at Hender-son Global Investors, said: "We were committed from the outset to get the views of as many stakeholders on the scheme and have been through a lengthy consult-ation to reach this stage.
"We are confident the plans which have been developed will create a first- class retail and leisure destination that will encourage further invest-ment from national and international brands alike."
GLASGOW is a shopping destination of local, national and international significance, ranking number two in the UK and number 18 in Europe.
Council leader Gordon Matheson said: "While I cannot comment on an individual planning applic-ation, any proposal which helps Glasgow to maintain its position as the UK's biggest shopping destination outside of London's West End is to be welcomed.
"The economic benefits of the regeneration projects across the city over the past few years are well docum-ented and such investments undoubtedly play a key role in our future prosperity."
Stuart Patrick, chief executive of Glasgow Chamber of Commerce, said: "The expansion of Buchanan Galleries will offer a vital contribution to the city centre, further reinforcing its status as the number one shopping district in the UK outside London.
"The development takes into account the continued need to ensure the city centre remains an attractive destination for visitors through its retail and leisure offering, as well as being a huge asset to the overall region's economy."
The new extension will almost double the size of the existing shopping complex.
The scheme will include £80m of public realm improvements to areas around the complex. They will be funded under a complex finance deal which allows the city council to borrow the cash over 25 years and recoup the money through additional business rates as a result of the Galleries expansion.
Scottish ministers provisionally agreed the scheme in April last year but attached 11 conditions.
Councillors agreed to the conditions. Buchanan Galleries bosses said the extension could only go ahead if the council funded the additional works.