Parent company Moran Hotel Group has announced that it now has planning permission to go ahead with the development, which will also add 81 new rooms.
Work on the project is due to start soon and is scheduled for completion in time to capitalise on the Olympics in 2012.
Moran Hotel Group owner Tom Moran said: "We're delighted to be able to announce good news and a much-needed boost to the local economy. With the London Olympics just around the corner, we'll be prepared for it with a great extension."
Architects from Project Orange, the company which designed the hotel prior to its opening in 2006, have been assigned to oversee the expansion.