Facebook has announced plans for a vast new extension to its California HQ designed by architect Frank Gehry. Initial plans for the site in Silicon Valley’s Menlo Park feature a 39,000 sq m single storey space, which claims to be the world’s largest open-plan office, with a tree-filled rooftop garden that aims to blend the structure into its surroundings. Founder Mark Zuckerburg announced the news in a status update, saying, ‘The idea is to make the perfect engineering space: one giant room that fits thousands of people, all close enough to collaborate together.’ The new site will sit across the road from the existing campus, joined by an underground tunnel, and house 2800 of the company’s engineers. Construction is expected to take two years beginning early 2013. The budget has not been disclosed.