On 8 October 2008, the embassy announced a conditional agreement with the real estate developer Ballymore Group to purchase property for a new embassy site on the South Bank of the River Thames in the Nine Elms area of the London Borough of Wandsworth. The site lies within the Vauxhall/Nine Elms/Battersea Opportunity Area as set out in the London Plan. The proposed plan would only go forward if approved by the United States Congress and by the local planning authority. The Northern line extension to Battersea is to have new stations at Battersea and Nine Elms—combined with major local development. The United States Department of State announced in January 2009 that it was choosing among nine architectural firms, all "modern" and "upmarket", to replace the ageing embassy headquarters. In March 2009, the U.S. Department of State's Bureau of Overseas Buildings Operations announced that four architectural firms had been selected for the final phase of the design competition. By law, the architect for a U.S. embassy must be an American firm with "numerous security clearances".