[Context and motivation]
Requirements engineering methods and examples presented in textbooks and scientific publications usually treat software which is developed - and therefore specified - from scratch. However, in the software development practice, this situation is very rare. In an industry case study, we encountered the situation that a software system in use had to be enhanced by a small delta.
Our objective was to specify these delta requirements without having to describe the complete system in detail. Therefore we explored how much of the existing system had to be specified in order to make the delta requirements understandable.
We made an intensive literature search to proven practices. As we were not successful we applied the requirements engineering method TORE and extended it to capture the delta requirements.
In this paper we describe a process for capturing delta requirements. To our knowledge, this is the first work about this practically relevant question. In our case study, hierarchical refinement of requirements top-down and iterative requirements prioritization successfully supported the specification of deltas, combined with a high-level specification of the existing system. We also present our experiences during the case study and propose ideas for further research.