In Australia, the dual occupancy concept was proposed by the Melbourne and Metropolitan Board of Works in 1981 to enable existing houses and housing lots to be subdivided into two to make more efficient use of existing facilities and urban infrastructure, to provide support links between the two households and to minimise external maintenance and gardening burdens. Nowadays, accessory dwelling units are increasingly being legitimized by state and local governments. These units can be a division of, addition to, or separate from the principal dwelling and are usually subject to minimum allotment sizes, site coverage, access and setback requirements. While dual properties are a fairly new concept in Australia, they have long been a common style of dwelling in countries such as Sweden, Japan and the United States.