Castellana 66 is a mixed-use office building erected in 1990 on Paseo de la Castellana, Madrid. The building currently is energy and carbon inefficient, primarily due to its substandard windows and insufficient thermal insulation. Past records of electricity bills also suggested that there is scope to retrofit the building for better energy efficiency and carbon reduction. Whilst the building is approaching the end of the lifetime, the challenge was to transform the structure into an efficient building good for the environment and good for people to propelled it to a new life.