From London to Lagos, “affordable housing” has become an oxymoron. In most cities, rents and home prices have increased faster than incomes, and in urban areas with robust job markets, housing stocks have failed to keep pace with demand. Some 330 million urban households either live in substandard housing, or pay more for their housing than they can afford. If current trends are not reversed, that number could grow to 440 million by 2025.