The major emphasis in developing design tools for web applications has been that of content. User requirements are often assumed. We would argue that the requirements are as significant for web applications as they are for any other application. Use case modelling is regarded as an ideal tool for capturing software requirements. This paper describes a requirements driven approach for generating the source code required for a web based application. Use case modelling is an ideal mechanism for determining web site navigability because it encapsulates all of the desired sets of user interactions with the web application. Using this it is possible to map user interactions to sets of request/response events and from there to a sequence of event trees for each Use Case. These event trees can then be used to automatically generate the code for the required web based application. The process described in this paper, which we call WEBGen, is used to determine a mapping between each use case scenario step and a set of request/response events. The number of possible request/response events is relatively small which makes the mapping between scenario steps and code a manageable task. WEBGen is demonstrated by using it to develop the code for an online practical registration system.