Environmental issues are increasingly getting concern in recent years and various environmental strategies are implemented in building level and city level. Green building or high performance building strategies is getting to be popular and enhanced in many cities today. The effect of wide spreading green buildings at the city level, however, is not clear from findings obtained from the perspective of individual buildings. In studying and planning effective building policies and measures, it is important to also consider the characteristics of cities and their infrastructures condition. In this light, we carried out case studies and quantified the environmental load reduction and economic effects of environmental measures taken at the individual building level to elucidate their effects on the city level. From among the various parameters of environmental performance, this analysis focuses on water consumption in buildings and its effects on urban water and sewage systems as one of examples. In this study, Tokyo 23 Special Wards are selected as case study cities. For the Tokyo 23 Wards, the estimates confirm that improving the environmental performance of houses in terms of household water consumption by water conservation and other means has the effect of not only reducing water consumption but also of reducing energy consumption and $CO_2$ emissions in the operation and maintenance of the water and sewage facilities systems of the urban infrastructure.