The third generation cellular systems, UMTS, will provide a new range of multi-media services supported by multi-rate transmissions. Based on W-CDMA technology, the increase of complexity has raised new challenges in the design of third generation (3G) mobile networks leading to a modification of the second generation (2G) system design. In order to reduce the network deployment costs, the instinctive approach for site acquisition is to re-use GSM cell locations as much as possible. However, the dimensioning approach for GSM and UMTS differs somewhat theoretically and in a practical sense. The capacity and coverage are in relation to the amount of interference within the UMTS network. This interference can increase with the introduction of multi-services supported by multiple bearer rates. In order to assess the validity of re-using the GSM cell locations, a UMTS system has been simulated using Motorola's internal static simulator. The network, located in a typical dense urban environment, has been simulated for a mix of services up to a 144 kbps throughput rate over 74 cells. This paper presents the results of the simulation, in terms of coverage, capacity and quality of service. The paper is organised as follows: the first section presents general aspects of the W-CDMA network planning. Section 2 describes the simulation modelling approach while the last section disseminates the results of a simulated UMTS network re-using a GSM cell sites configuration.