Smartphones show potential to improve and support the daily lives of people with disabilities. User experience of mobile apps is generally good, but rather unsatisfying for people with disabilities relying on accessibility support. Developers often are not aware of and do not respect this user group. We propose a system supporting developers in early stages of app development. The Accapto toolkit allows modelling accessible smartphone interaction during the design phase. Based on this interaction model the prototype of the resulting app is generated. Predefined accessibility features based on standards and expert knowledge are automatically integrated to enhance accessibility.