The building sector contributes a substantial amount to the overall energy consumption worldwide along with a large share in the corresponding greenhouse gas emissions. Thus, improving the performance of buildings is vital to achieve the energy and environmental goals. In this regard, Denmark is not an exception, where the building sector was prioritized, aiming to enhance newly-built buildings’ performance along with upgrading existing buildings through a comprehensive energy retrofit strategy. This study aims to present and demonstrate a decision-making tool for energy retrofit design and assessment of Danish buildings (DanRETRO). Unlike the current energy retrofit assessment methodologies and tools used in the Danish building market, DanRETRO builds on a database comprising a large number of simulations for Danish buildings’ performances of various types, sizes, and ages. The well-established modeling and simulation engine of EnergyPlus is used to develop the dynamic energy models. The DanRETRO tool development is presented, where multiple building retrofitting techniques and measures are carried out along with assessment of the impacts of implementing these improvements on the technical, economic, and environmental levels. The tool’s demonstration in three case study buildings is presented, where the retrofit assessment results are reported and evaluated at various levels.