Property giant British Land is to purchase Marble Arch House in London's West End.The company is paying the present owners, The Portman Estate, £18m for the development site, which currently consists of two mixed use five story buildings on the corner of Seymour Street and the Edgware Road.The new building will comprise 60,000 square feet of office accommodation together with 15,000 square feet of retail space. Adjacent to the new office building will be a development of ten luxury residential apartments with a 5,000 square feet restaurant unit at ground level built behind the retained period Victorian facade of 62-64 Seymour Street. The total cost of development (excluding land and interest) is expected to be around £40m.Demolition and ground works are expected to start in mid-2011 with practical completion scheduled for mid-2013. On completion of the development, British Land will be granted a new 125 year leasehold interest on the commercial element of the development and a separate long leasehold on the residential units.