The impact of climatic change through greenhouse gas emission is a recognized major global crisis. Energy use in residential and commercial buildings is a major part of the total consumption in European countries, and is estimated to be about 40% of the total load. Currently the concept of building retrofit has become a top priority for the UK government in order to meet the national plans for reducing CO2 emissions by 80% compared to 1990 levels by 2050. This study presents the simulation results for a case study of energy and CO2 emission savings of a 19th century semi-detached building in the UK. The building was refurbished to high standards of energy efficiency, with four simulation scenarios developed for analysis: As built, As built 1965, As reality and post retrofit. DesignBuilder software was used to simulate the annual energy consumption and carbon emissions in all cases. In addition to this, thermal imaging and air tightness tests were conducted and the results were used to validate the models. The post retrofit results showed there is a significant reduction in energy consumption that exceeded 80% with carbon emissions being reduced above 70 %. Economic analysis of each retrofit scenario was then undertaken, and results showed payback varied between 9 to 40 years due to the fluctuation in fuel prices and construction retrofit materials. The models indicate performance of the building post retrofit can be significantly improved in terms of energy reduction and CO2 emission savings. Further research is being performed to improve performance through field monitoring and installation of innovative retrofit technologies.