The study is based on the analysis of survey record of around 50 houses of different rooftop areas of Junagadh city. The estimation of the appropriate size of the water tanks and their costs required to fulfill the annual drinking water demands through Domestic Rooftop Water Harvesting (DRWH) from rooftop of different areas are done. A mathematical equation expressing the relationship between the required size of water tank and different rooftop areas is developed. The DRWH systems for all houses are designed considering the existing rain water outlets and cost estimation for each individual house is done. A cost model expressing the relationship between rooftop area and cost of DRWH system is developed.