Decent, affordable housing is fundamental to the health and well-being of people and to the smooth functioning of economies. Yet around the world, in developing and advanced economies alike, cities are struggling to meet that need.A new report by MGI shows that the affordable housing gap now stands at $650 billion a year and that the problem will only grow as urban populations expand: current trends suggest that there could be 106 million more low-income urban households by 2025, for example.However, four approaches used in concert could reduce the cost of affordable housing by 20 to 50 percent and substantially narrow the affordable housing gap by 2025.They are Unlocking Land supply, Reducing construction costs,Improved construction and maintenance,Lowering financing costs for buyers and developers.