Software customization has been argued to benefit both the productivity of software engineers and end users. However, most customization methods rely on specialists to manually tweak individual applications for a specific user group. Existing software development methods also fail to acknowledge the importance of different kinds of user skills and preferences and how these might be incorporated into a customizable software design. This paper proposes a framework for performing requirements analysis on user goals, skills, and preferences in order to generate a customizable software design. We illustrate our methodology with an email system and review an on-going case study involving users with traumatic brain injury.