Section 603 permits buildings to comply with energy code by meeting a calculated energy cost budget, rather than complying with prescriptive requirements. Building energy is simulated using approved software that models heating, cooling, lighting, and plug loads. Building performance (actual efficiency) is compared to a baseline building meeting prescriptive requirements. If the building uses equal or less energy than the baseline, compliance is demonstrated. This method permits trade-offs, such as using a less efficient HVAC system if compensated by superior building envelope insulation.