This data on the number of total building permits and building permits for single-family homes comes from the SOCDS Building Permits Database which "contains data on permits for residential construction issued by about 21,000 jurisdictions collected in the Census Bureau's Building Permits Survey (Documentation available here.) ."
“The purpose of the Building Permits Survey is to provide national, state, and local statistics on new privately-owned residential construction. The United States Code, Title 13, authorizes this survey, provides for voluntary responses, and provides an exception to confidentiality for public records. Over 98 percent of all privately-owned residential buildings constructed are in permit-issuing places. Data collected include number of buildings, number of housing units, and permit valuation by size of structure. Data on permits issued for new construction are available monthly and annually since 1959; reported data are for permit issuing activity taking place during the applicable reference period."
Economists use index numbers for quick comparisons between sets of data across time. The Building Permits, Single Family (Indexed to 2005) measure is calculated by identifying 2005 as the "base year" and scaling all of the other ‘Building Permits , Single Family’ results off of that year.
For example, the ‘Building Permits - Single Family’ value for Alabama in 2005 is represented by 1.0 (obtained by dividing the value by itself, 24132/24132 = 1.0) and the numbers from other years are similarly scaled down. The ‘Building Permits - Single Family: 2005 = 1.0’ value for 2018 is 0.53 obtained by dividing 2018 ‘Building Permits - Single Family’ value by the base year’s ‘Building Permits - Single Family’ value ( 24678/24132 = 0.4729)