The online fashion retailer said it has taken on an additional 40,000 sq ft to house the extra workers and will spend £40m on renovating the space in Camden.
ASOS currently employs 2,500 people at Greater London House.
Boss Nick Beighton said: "Our people are what make ASOS special. We are creating a workspace that fosters creativity, where they can enjoy what they do.
"Greater London House is such an important part of the company's history, as well as Camden's, so I'm thrilled that we get to stay in our home, while building a workplace for the future."
ASOS has recently been dogged by claims surrounding working conditions at its warehouse in Barnsley, South Yorkshire.
But the company has consistently said the allegations are "inaccurate and misleading."
The new Camden space, designed by architects Spacelab, will boast health and workout facilities, an ASOS training academy, a quiet zone library, cafes and eating areas and an auditorium for company-wide gatherings.
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