; and (B) guidelines for energy-efficient building design. (2) Targets The stretch codes and advanced standards shall be designed— (A) to achieve substantial energy savings compared to the model building energy codes; and (B) to meet targets under section 307(b), if available, at least 3 to 6 years in advance of the target years. (h) Studies The Secretary, in consultation with building science experts from the National Laboratories and institutions of higher education, designers and builders of energy-efficient residential and commercial buildings, code officials, and other stakeholders, shall undertake a study of the feasibility, impact, economics, and merit of— (1) code improvements that would require that buildings be designed, sited, and constructed in a manner that makes the buildings more adaptable in the future to become zero-net-energy after initial construction, as advances are achieved in energy-saving technologies; (2) code procedures to incorporate measured lifetimes, not just first-year energy use, in trade-offs and performance calculations