The function of water distribution system, which is one of the important infrastructures of modern society, should always remain reliable under any circumstances. The system can be expressed as water distribution networks (WDNs), a two-dimensional complex network consisting of nodes (e.g., source and demand) and edges (e.g., pipes). WDNs have the composite of lattice-like and tree-like structures and can be evaluated by using these characteristics. The evaluation indicated that the function of the current network is inefficient, and the grid ratio is also significantly below the threshold. The function of the system can be enhanced by increasing the proportion of the grids of real WDNs. However, at a fixed grid ratio, the WDNs can have multiple structures, and the efficiency varies accordingly. Therefore, it is desirable to install new grids at critical points where the function of the WDNs can be improved the most. Dual mapping approach and primal mapping approach both can be used to determine critical locations. In addition, since there are spatial constraints in real WDNs, it is necessary to consider pipe length distribution of current WDNs. Using various layers (e.g., road layers) based on GIS can also help determining the possibility of pipe addition. The purposes of this study are to (1) identify reliable locations where pipes can be added, (2) rank the identified locations where pipes are added regarding the extent of functional increase, and (3) propose an optimal network design with increased grid ratio. This approach proposes a new framework for structural modification of the network by increasing the grid ratio of existing WDNs and helps reducing analysis time for distinguishing reliable locations for pipe additions.