“Raising London’s population density may not be as hard as it looks, because, despite its crowded tubes and roads, London is sparsely populated. The typical number of dwellings per hectare is 1,700 in Kowloon, 500 in Barcelona and 300 in Paris. In inner London, even recent developments have averaged only 78. If London had the population density of Haussmann’s Paris, it would house not 7.5m people but 35m.”