Research always is researching what we don't know. So the most important in any academic thesis is the formulation of an original research question or problem. The second step is not on "existing theory", but doing a literature review directly related to your research question. Not an encyclopedic summary of literature and theories on a specific scientific field. The literature review is a key process that must identify a gap that corresponds to your research question as well as determining the data already available on the problem that becomes secondary data. Then, it is expected that you collect original data regardless of the amount of secondary data and its available conceptualizations. From this, you need to build up a descriptive, analytic and conceptual interpretation that becomes a theorization (an explanation using scientific concepts...) answering to your research question. This process must be original, "new", and if finally, it matches, confirms, informs, or contradicts existing theories will be your research conclusion. Case studies have some theoretical and methodological conditions: they must be chosen due to their exclusive singularity or high degree of representation of subjects (as populations, communities, especial subjects as drug addiction, domestic violence, etc.) or objects (an industrial product, food, drink, clothes, a painting, etc.). It is not a valid research option to label a case study for any singular investigation: one is not synonymous with a case study. If you are using the epistemic and methodic framework of a case study in which the data is collected, as you suggest, through surveys and interviews, these techniques must concretize an inquiry related to your research question and not any previous data or theory.