This paper discusses a web-based tool that allows members of a design team to collaboratively specify a hierarchical spatial program for an architectural project. Given its object orientation, the represented artifacts have built-in data and methods that allow them to respond to user actions and manage their own sub-artifacts. Given that these components are hierarchical allows users to filter information, analyze and compare design parameters and aggregate hierarchical amounts in realtime. Furthermore, the software goes beyond outlining functions to support synchronous collaborative design by linking each item in the spatial program to a detail page that allows file uploading, realtime group marking of images, and textual chat. Thus, the software offers a seamless transition from the largely asynchronous definition of an architectural program to synchronous collaboration. In addition, and in contrast to commercially available groupware, the software allows multiple collaboration sessions to run at the same time. These sessions are artifact-based in the sense that they get automatically initiated once participants visit the same architectural space in the program hierarchy. The software employs a three-tier object-oriented, web-based scheme for a richer representation of hierarchical artifacts coupled with a relational database for server-side storage. The prototype integrates this technology with Java-based tools for synchronous web-based collaboration.