There are numerous forecast models of software development costs. However, various problems become apparent when it comes to practical application. Standardized methods, such as COCOMO II for instance, have to be calibrated at an individual operational level on the basis of the underlying database. The following paper deals with a new activity based approach that is based on business specific cost data and that can be easily integrated into existing management accounting systems. It can be applied for software development projects based on the unified process. Activity driven budgeting promises several advantages compared to common tools in use. It supports enterprise specific cost forecasting and control. Furthermore, it can be easily linked with risk analysis. Besides the presentation of a conceptual design model this paper delivers a framework for activity driven budgeting and cost management of software development projects in general combined with concrete implementation examples.