Indoor water use is calculated separately for residential and commercial buildings. Annual residential indoor water use is calculated by applying the per-capita water use rates to the population of dwelling units of each of the four housing types: large lot detached single family units (usually lot sizes greater than 5,500 sq. ft.), small lot detached single family units (usually lot sizes under 5,500 sq. ft.), attached single family units (townhouses), and multifamily units. UrbanFootprint uses 5,500 sq. ft. as the default cutoff between small and large lots. Annual indoor water use is calculated as the product of the water use rate associated with each housing type in a given scenario, the population by housing type, an annualization factor, and a variable conservation factor.