Building codes currently regulate energy efficiency in newly constructed buildings in Sweden. Alongside energy declarations, performance-based regulation specifying specific energy use requirements was introduced in Sweden in 2006. The requirements have been subsequently tightened to enhance energy performance. This study estimates the impact of these requirements on energy savings in Swedish multi-apartment buildings, relying on specific energy use data from energy performance certificates (EPCs). An estimated time trend indicates greater energy efficiency at a rate of 1.57% per year for buildings with district heating and 1.09% per year for electrically heated buildings. After accounting for this trend, the results indicate that the implementation of performance-based regulation is associated with a 14.2% increase in energy efficiency for buildings with district heating and a 9.7% increase for those with electric heating. Moreover, the first tightened building codes generates an additional 2% increases in energy efficiency for district-heated buildings and an approximately 7.4% improvement for electrically heated buildings. However, there is no evidence to suggest that the second tightening of building codes have strong effects on further increasing energy efficiency. Furthermore, the effect of building codes is more substantial for buildings where actual energy use exceeds the mandated levels and modest for buildings with better energy performance. Alternatively, when studying the time trend of energy efficiency, I find a structural break with a significantly greater increase in efficiency over time during the period of regulation compared to before. In addition, findings in this study indicate evidence of the energy performance gap, where the estimated energy use from engineering models is substantially lower than the measured energy use for comparable construction.