Sustainability has become a critical element of housing development in large metropolitan areas around the world. Since NYC's resident population is expected to reach 9 million, the city began to make significant efforts to build a sustainable housing stock. One of them is to focus growth on the areas well served by mass transit and control population density in car-dependent neighborhoods. Since 2007, nineteen transit-oriented rezoning projects have been adopted in NYC. These rezonings will result in creation of additional 35,000 housing units. Most of these rezoning efforts preserve the scale of existing neighborhoods, while focusing growth along key corridors that can best accommodate future growth. The City also capitalizes on opportunities to develop underused sites. One such project is the Caemmerer Railyards that will transform a 26-acre MTA/Long Island Rail Road train storage yard on the far west side of Manhattan into a high-density residential and commercial complex at the focal point of the Hudson Yards. Planned extension of the #7 subway line will provide the new development with convenient public transportation. In order to ensure sustainable growth, NYC also directs a mix of new market-rate and affordable housing units into the areas served by mass transit. Mixed-income neighborhoods offer a wide range of options for employment, shopping, and services accessible on foot or by transit. It also encourages environmentally conscious and healthy alternatives to car ownership such as walking, bicycling, and car sharing and development of pedestrian-friendly streets. These measures can improve overall air quality and reduce greenhouse gas emissions. The result of such strategies as rezoning and development of underutilized sites is that 87 percent of all housing units created since 2007 are within a half mile of transit. By comparison, only 69 percent of the city's housing units were in such proximity to mass transit prior to 2007. Energy efficiency is another important aspect of sustainable housing. While the rising utility costs are out of the city's control, and in fact between 2005 and 2008 they contributed more to the total cost of living in New York than rising rents, some measures are still be taken to lower energy consumption (for more detail, see the Energy section of this chapter). On October 3, 2005, NYC adopted Local Law 86 (LL86), one of the nation's first green building laws. It requires many capital projects that involve building construction and that receive city funds to be built in accordance with the rigorous standards of the Leadership in Energy and Environmental Design (LEED®) green building rating system. It requires that most of these projects, as well as larger Heating, Ventilating, and Air Conditioning (HVAC) systems, lighting and plumbing upgrades be designed to reduce the use of energy and potable water well below the standards prescribed by the current NYC building code. Also beginning in fiscal year 2011, all new construction projects financed by Housing Preservation and Development (HPD) will be required to achieve Green Communities Certification, which provides affordable housing providers with cost-effective standards for creating healthy and energy-efficient homes. These measures will contribute to a cleaner and healthier NYC and will reduce our carbon footprint.