Affordable housing is housing which is deemed affordable to those with a median household income or below as rated by the national government or a local government by a recognized housing affordability index. Most of the literature on affordable housing refers to mortgages and number of forms that exist along a continuum – from emergency shelters, to transitional housing, to non-market rental (also known as social or subsidized housing), to formal and informal rental, indigenous housing, and ending with affordable home ownership.In Australia, the National Affordable Housing Summit Group developed their definition of affordable housing as housing that is "...reasonably adequate in standard and location for lower or middle income households and does not cost so much that a household is unlikely to be able to meet other basic needs on a sustainable basis." In the United Kingdom affordable housing includes "social rented and intermediate housing, provided to specified eligible households whose needs are not met by the market."
The notion of housing affordability became widespread in the 1980s in Europe and North America. In the words of Alain Bertaud, of New York University and former principal planner at the World Bank,"It is time for planners to abandon abstract objectives and to focus their efforts on two measurable outcomes that have always mattered since the growth of large cities during the 19th century’s industrial revolution: workers’ spatial mobility and housing affordability."Housing choice is a response to an extremely complex set of economic, social, and psychological impulses. For example, some households may choose to spend more on housing because they feel they can afford to, while others may not have a choice.