This paper presents a development approach and design of a task-centered agent-based model (ABM) to represent the interactions of occupants with a commercial office building. The model is built with the understanding that occupant behaviour is driven by tasks the occupant performs. A contextual task analysis questionnaire explored occupant perspectives on the interactions between their tasks, their individual behaviour and comfort, and the physical characteristics of their workspace. This task-based information defines five ABM elements that represent occupants, task and workspace environment, task list, occupant actions, and the impact of the occupant-workspace interaction on tasks. An example of an occupant, performing a task, and conducting an action in response to an environmental mismatch demonstrates the ABM design. The example discusses the generation of possible actions as well as the result from those actions in terms of task performance and occupant satisfaction. As the ABM design evolves, it will aid in the understanding of occupant behaviour in buildings, and ultimately standardize the approach to occupant behaviours affecting building energy demand.