Because of supply and demand, the most "affordable" places are where there is the least demand relative to supply. Where the supply of available housing is less than the demand, low- and moderate-income households often struggle to obtain housing that is affordable. In these housing markets, rising land values often outpace rising incomes. Such housing markets often have a limited supply of residential land, or a number of regulations that make it difficult or costly to increase housing supply at rents affordable to consumers at income ranges below the local average.
Measuring demand is complicated, and subject to different views. It can be measured in terms of the costs for housing, housing type (such as apartments vs. single-detached homes, or the size and configuration of units, including number of bedrooms) and location for housing (relative to commercial/employment centers, transportation infrastructure, schools and other community resources.) A key element in measuring housing demand is differentiating between the "ability to pay" that some households have, and the "willingness to pay" of households for certain housing types in certain locations. When a place has attributes that trigger high degrees of "willingness to pay", prices often rise due to the finite supply, thereby changing that place's relationship to household "ability to pay". When a place has attributes that make it undesirable, the willingness to pay is reduced and the price falls. This explains why some places within an otherwise unaffordable area (measured in the aggregate) remain very affordable, such as a distressed inner city neighborhood in an otherwise expensive city.
In the United States, a key element in determining affordable housing is acceptable commuting time/distance. In Southern California, for example, a household's inhabitants must decide whether to pay more for housing to keep commuting time and expense low, or to accept a long and/or expensive commute in order to obtain "better" housing.