HS2 Ltd is overseeing the expansion of Euston to cater for passengers using the first phase of the new rail line between London and the Midlands.
The revamp was originally scheduled to end in 2026 but the firm now says it will finish in 2033.
Camden Council has warned the extended works will "blight" the area, but HS2 says it will be "easier to manage".
The station is set to gain six new platforms and an additional concourse, with building works due to start in 2017.
In a change to plans submitted two years ago, HS2 Ltd has proposed the construction of five further platforms within the existing station should be delayed.
They would be ready for phase two of HS2 which will run from the capital to Leeds and Manchester in 2033.
Rupert Walker, Euston development director of HS2 Ltd, said: "It is more construction over a longer period but it's less intense. It will be easier for us to manage."
The impact of building all 11 platforms in one go would have caused a reduction in the capacity of trains using Euston every day, he said.
Transport Secretary Patrick McLoughlin said the changes would "dramatically improve the design" of the station.
Sarah Hayward, leader of Camden Council, predicted: "HS2 will cause decades of blight in the Euston area - to property prices, to our small business' trade and to our residents' lives."
She said the council is "ardently opposed to the scheme", adding it could only be a success with "close collaboration" between the local authority, residents and businesses.
HS2 Ltd's new plan will be submitted to Parliament next week in order to form part of the legislation which will allow phase one of High Speed 2 to go ahead.
The firm also announced that the budget of the Euston project has risen from Â£2 billion to Â£2.25 billion, but said this would not affect HS2's overall budget of Â£42 billion.
The redesign of London's Euston station to accommodate High Speed 2 will take seven years longer than planned.