From a user-centred perspective, this study focuses on how to increase usability through design principles and usability aspects and how to achieve it on a website with a heterogeneous target group. This is a case study since a specific website has been chosen. We are focusing on usability regarding age and therefore we have chosen a website that holds a wide range of different users. The method for our qualitative research was made by interviews, and the results were analysed through a thematic analysis. We noticed that the main part of the respondents pointed out that the website lacks many different design principles and usability aspects in order to be considered useful. The result of the thematic analysis concludes the fact that Visibility, Constraints and Consistency are the most important design principles when developing websites for a heterogeneous target group. According to the thematic analysis, our respondents seem to think that the most important usability aspects are Satisfaction and Efficiency to accomplish good usability. Based on the analysis, we interpret those aspects and principles both are required to prior knowledge.