Buildings with all their activities use considerable energy in the urban environment. Environmentally friendly buildings concept becomes essential with energy conservation. The application of this concept to existing buildings can be made through retrofit, by carrying out a series of improvements or changes to facilities and elements, to reduce energy consumption. This research was conducted to investigate how much energy savings will be generated through the simulation of several retrofit plans on the building envelope. The model in this study is the public office building, Jakarta Provincial Government’s office. The projects are carried out by simulating the reduction of Window Wall Ratio, the addition of shading, and the replacement of glass material. The analytical method used is energy simulation with sensitivity analysis using the Autocad Greenbuilding Studio application. From the simulation, the results showed that retrofit plans proved to be able to save energy use in the building by 18 - 22%. This simulation also affects the savings in operational costs.