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