constantly being developed to complement current practices in creating greener structures, the common objective of green buildings is to reduce the overall impact of the built environment on human health and the natural environment by: Efficiently using energy, water, and other resources Protecting occupant health and improving employee productivity (see healthy building) Reducing waste, pollution, and environmental degradation Natural building is a similar concept, usually on a smaller scale and focusing on the use of locally available natural materials. Other related topics include sustainable design and green architecture. Sustainability may be defined as meeting the needs of present generations without compromising the ability of future generations to meet their needs. Although some green building programs don't address the issue of retrofitting existing homes, others do, especially through public schemes for energy efficient refurbishment. Green construction principles can easily be applied to retrofit work as well as new construction. A 2009 report by the U.S. General Services Administration found 12 sustainably-designed buildings that cost less to operate and have excellent energy performance. In addition, occupants were overall more satisfied with the building than those in typical commercial buildings. These are eco-friendly buildings. Reducing environmental impact Buildings represent a large part of energy, electricity, water and materials consumption. As of 2020, they account for 37% of global energy use and energy-related CO2 emissions, which the United Nations estimate contributed to 33% of overall worldwide emissions. Including the manufacturing of building materials, the global emissions were 39%. If new technologies in construction are not adopted during this time of rapid growth, emissions could double by 2050, according to the United Nations Environment Program. Green building practices aim to reduce the environmental impact of building as the building sector has the greatest potential to deliver significant cuts in emissions at little or no cost. . General guidelines can be summarized as follows: Every building should be as small as possible. Avoid contributing to sprawl, even if the most energy-efficient, environmentally sound methods are used in design and construction. Bioclimatic design principles are able to reduce energy expenditure and by extension, carbon emissions. Bioclimatic design is a method of designing infrastructure within the context of their respective environment while using the surroundings to advantage whenever possible. This could be as simple as constructing a different shape for the building envelope or facing the building towards the south to maximize solar exposure for energy or lighting