5 million visitors a year visit the National Gallery for free, but have to put up with steep steps, cramped foyers and a basement café dating from the 1950s. All the existing main galleries are raised well above street level - but there is a large amount of forgotten space beneath them and in hidden spaces between them, which will be opened up. As with Tate Britain, the Royal Academy or the neighbouring National Portrait Gallery - all of which have had similar internal makeovers in recent years - the National Gallery will conjure new space out of thin air.