The FHFA HPI is based on transactions involving conforming, conventional mortgages purchased or securitized by Fannie Mae or Freddie Mac. Only mortgage transactions on single-family properties are included. Conforming refers to a mortgage that both meets the underwriting guidelines of Fannie Mae or Freddie Mac and that does not exceed the conforming loan limit. For loans originated in the first nine months of 2011, the loan limit was set by Public Law 111-242. That law, in conjunction with prior legislation, provided for loan limits up to $729,750 for one-unit properties in certain high-cost areas in the contiguous U.S. Mortgages originated after September 30, 2011 were no longer subject to the terms of prior initiatives and, under the formula established under the Housing and Economic Recovery Act of 2008, the "ceiling" limit for one-unit properties in the contiguous U.S. fell to $625,500. For 2018-acquired loans, the ceiling limit rose to $679,650 for one-unit homes in the contiguous U.S.

Conventional mortgages are those that are neither insured nor guaranteed by the FHA, VA, or other federal government entities. Mortgages on properties financed by government-insured loans, such as FHA or VA mortgages, are excluded from the HPI, as are properties with mortgages whose principal amount exceeds the conforming loan limit. Mortgage transactions on condominiums, cooperatives, multi-unit properties, and planned unit developments are also excluded.