The internationally renowned British architect puts substance before image, and isn't interested in a building's iconic presence on a skyline. 'How many squiggles can a city take?' he once asked. He has been described as classical, minimalist, simple, but if there is a word he would like to apply to his architecture, it is 'humane'. Alan Yentob talks to Chipperfield about his breakthrough in Berlin, his love of the city and its history and the 11 years spent on the transformation of the Neues Museum, his 'masterpiece'. After successes at the Hepworth Gallery, Wakefield, and Turner Contemporary Margate, he is now embarking on his most prestigious project ever, a new gallery for the Metropolitan Museum of Art in New York.