This paper presents a heuristic approach for the optimum layout design of multi-span flat slab floors in order to optimise the total cost. For this purpose, a heuristic methodology is developed in order to achieve a new objective function for the structural optimisation problem. The proposed objective function has the capability to make use of action effects of the structure as the alternative design variables in place of the commonly used cross-sectional ones. Such a feature provides the method with the ability to be easily employed in large and realistic structural optimisation problems. Furthermore, the proposed formulation, in an iterative optimisation process, takes less time than its counterparts as it uses the action effects that can be easily obtained from a structural analysis procedure instead of the cross-sectional variables that are typically the results of a design procedure.