still immobile). Consumers come to the good rather than the good going to the consumer. Because of this, there can be no physical marketplace. This spatial fixity means that market adjustment must occur by people moving to dwelling units, rather than the movement of the goods. For example, if tastes change and more people demand suburban houses, people must find housing in the suburbs, because it is impossible to bring their existing house and lot to the suburb (even a mobile homeowner, who could move the house, must still find a new lot). Spatial fixity combined with the close proximity of housing units in urban areas suggest the potential for externalities inherent in a given location. Housing industry The housing industry is the development, construction, and sale of homes. Its interests are represented in the United States by the National Association of Home Builders (NAHB). In Australia the trade association representing the residential housing industry is the Housing Industry Association. It also refers to the housing market which means the supply and demand for houses, usually in a particular country or region. Housing market includes features as supply of housing, demand for housing, house prices, rented sector and government intervention in the Housing market. Demand for housing The main determinants of the demand for housing are demographic. But other factors, like income, price of housing, cost and availability of credit, consumer preferences, investor preferences, price of substitutes, and price of complements, all play a role. The core demographic variables are population size and population growth: the more people in the economy, the greater the demand for housing. But this is an oversimplification. It is necessary to consider family size, the age composition of the family, the number of first and second children, net migration (immigration minus emigration), non-family household formation, the number of double-family households, death rates, divorce rates, and marriages. In housing economics, the elemental unit of analysis is not the individual, as it is in standard partial equilibrium models. Rather, it is households, which demand housing services: typically one household per house. The size and demographic composition of households is variable and not entirely exogenous. It is endogenous to the housing market in the sense that as the price of housing services increase, household size will tend also to increase. Income is also an important determinant. Empirical measures of the income elasticity of demand in North America range