This paper proposes an optimization model to determine the optimal tank size of a single residentialhousing unit for rainwater harvesting and storage. Taking into account the site specific data such asthe rainfall profile, the roof area of the building, the water consumption per capita and the number ofresidents, an integrated optimization model based on linear programming is proposed to decide on thesize of rainwater storage tank to build such that the net present value of the total tank constructioncosts and freshwater purchase costs is minimized. The proposed model was tested on a case study fromNorthern Cyprus, the results of which emphasized the feasibility of rainwater harvesting as a sustainablesupplement to the depleting aquifers in the region. The study also offers managerial insights on the impactof various parameters such as the number of residents, roof area, discount rate, water consumption percapita, unit cost of building the rainwater tank, and rainfall characteristics on the optimal tank size andon the net financial benefit gained from rainwater harvesting through detailed sensitivity analysis.