Impact assessment (IA) is a key method for the legislator to evaluate policies, norms or regulations currently under development. Experts use IA to gather and analyze input from many individuals to obtain clear problem statements, estimations regarding policies etc., and use this information to compare policy alternatives. Currently, the opinions, expertise etc. gathered for IA need to be structured by hand. Thus, the analysis steps of IA are time-consuming, and IA does not scale with the number of persons involved. In this paper, we introduce a collaborative approach for IA. Based on a Web 2.0 architecture, we let a community of individuals derive the potential, downsides and design alternatives of policies collaboratively. An important characteristic of our approach is that it guides individuals through the process of creating structured input. Our approach is fully implemented, and we have evaluated it together with legal experts using the structured-case method. While our evaluation reveals that the acceptance of web-based IA strongly depends on the user interface, it also acknowledges that our approach can be an important tool for future IA.