This paper presents general guidelines for the required cooling capacity of an entire office building using thermally activated building systems (TABS). By activating the thermal mass of the building using pipes embedded in the floor, peak loads can be reduced. On-site measurements were performed to obtain the required cooling power of an entire building as well as individual zones. Beside this, the internal climate conditions of rooms and surface temperatures of the TABS were measured. The measured data were used to analyze the predictive performance of a simulation model. In order to acquire general guidelines for the required cooling capacity of a standard office building, simulations of an entire building were used to determine the impact of variable internal heat gains and different sized windows. The required cooling capacity was compared to the cooling capacity of a system without energy buffering (e.g. chilled ceiling panels). It was found that reductions up to 50% of the cooling capacity for a chiller can be achieved using TABS. The presented results within this paper can be used as design guidelines in the first stage of a design process. The results focus on temperate climates and were derived using Dutch climate conditions.