Energy efficiency retrofitting of existing buildings was long ignored by public authorities who favored energy efficiency policies in new buildings, which are easier to implement. It has now become indispensable in all economies "even emerging countries" given the constraints imposed by energy security and climate change, and because it represents considerable potential energy savings. This document, based on the experience acquired during a three-year applied research program conducted by the AFD and Hubei Province in China, describes the methodology used through a step-by-step illustration of its application in a real case. This methodological framework can of course be applied to other regions or other countries.