The Swiss Energy Strategy 2050 strongly relies on massive reduction of the heat demand for the building sector (45% by 2035, and 64% by 2050). Given the slow transformation of the building stock, focus needs to be set on building retrofit. Understanding and reducing the performance gap between actual (real) and planned (design) energy performance is an essential step towards achieving the energy saving goals. The objectives of this study are: i) Analysis of the energy performance gap in building retrofit on the basis of a representative set of case studies, identification and characterization of the determinant factors and estimation of their impact on the energy performance via sensitivity analysis; ii) Estimation of the actual energy saving potential of the building stock, under current practice; iii) Provide guidance on countermeasures and policy recommendations to reduce the energy performance gap.