3G UMTS networks are being widely deployed by several cellular operators worldwide that already have existing GSM networks. In this paper, we model the problem of GSM/UMTS co-siting as a nested mixed integer optimization problem which takes as input an area of interest with existing GSM sites and obtains as output the optimal number and locations of UMTS sites to be deployed (new and co-sited with GSM sites) in order to optimize target objectives. The goal is to minimize the cost of UMTS network deployment by reusing as many existing GSM sites as possible while guaranteeing that every cell is covering a target capacity load which is a percentage of the downlink pole capacity. We propose an algorithm taking into account both integer and continuous variables to solve the formulated optimization problem as a function of the user distribution and the existing fixed GSM site locations. Results for different scenarios show that it is optimal to reuse a subset of the existing GSM sites in addition to deploying new UMTS sites.