The link between urban form and building energy demand is a complex balance of morphological, constructive, utilization and climatic factor. Especially in the European compact city, where existing areas prevail on much more energy-efficient new settlements, it is evident that operative ways to transform efficiently the building stock have to be found. This paper explores the existence of a relation between built mass and energy demand depending on urban form. Focusing on the compact city of Mediterranean climate, tests on different case studies simulations are carried out. Results presented and discussed, point out that mass has strong relevance on energy demand and plays an important role in reducing energy consumptions. This paper is a preliminary report of an ongoing research study about one possible way to comprehend “metabolic rate” scaling law - the relationship between power and mass of a complex system in its process - concerning urban fabric. This knowledge-base could help verify the accordance with this rule on urban scale and give hints to conscious and effective built environment transformations towards more efficient conditions.