Here we document the design method of an air-based thermally activated building system (TABS) suited for the retrofitting of tertiary buildings, for cooling purposes mainly. The first phase of this work provides a general design and checks its consistency with the specifications of tertiary buildings by means of basic energy balances. Second, a numerical model of both the TABS and the room is developed under a finite element method multi-physics environment to better estimate the transient heat transfer for the proposed retrofitting solution. This results in the specifications for building at 1:1 scale prototype whose construction is documented.