This research document is developed to assess the energy flexibility potential in office buildings in Denmark. The study focuses on ability of building thermal mass to store energy. There are office and administrative buildings currently used in the country which are erected over different periods of time. They differ in thermal properties, heat capacity of building elements, air permeability, people load, equipment load, etc. Their building energy flexibility is expected to perform differently, therefore is examined, and classified. Four main building cases are included in the research, each of them representing specifications of building structure typical for their time of erection. Different configurations of internal loads and solar gains are assigned for each case. Assessment is conducted using a novel system which has the ability to both heat and cool by using the principle of forced convection. A flexibility controller, based on electricity price, is applied to the system in order to activate the thermal mass of chosen structures. Evaluation of investigated building models is done according to developed metrics.