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7 August 2013, 11:48
Mayor of London Boris Johnson has laid one of the last paving stones in the 25,000 sq ft square.
The development is so big it's been given its own postcode.
Graham Goodwin is from Network Rail and has told Heart it's a huge project. He said: "You could lay down the London Eye in the square and it wouldn't overlap the road on any side.
"It's a huge new space. It's something that possibly we wouldn't have done at any other point in history, which is to give space back to the public. It will actually let people see the front of the station."
The public space and improved entrances to the underground will officially open on 26th September.
Mr Johnson said: "The incredible regeneration of King's Cross continues apace.
"Network Rail are renovating this historic location with ingenuity and flair and this wonderful new square will be the perfect gateway to almost 70 acres of new public spaces, businesses and homes for Londoners, setting a new standard that we should aspire to for all future station redevelopment."