The improvement of existing buildings had been argued to be a major approach for the attainment of sustainability in the built environment, especially in the developing countries. However, literature had shown that users' requirement, which are hardly met in improved buildings, is a basis for attaining sustainable improvement. It was estimated that 87% of existing buildings will still stand by 2050, which therefore goes without saying that existing buildings require effective sustainable improvement that will satisfactorily reflect end-users' requirement, particularly in developing countries. The book thus approached sustainable improvement of existing buildings through the lean thinking concept with particular reference to public office buildings in Nigeria, a developing countries; this involves the application of muda to the typical improvement diagnosis technique for enhanced assessment of users' requirement and consequently enhanced building performance for sustainability.