Self-determination is a universal right and a central concept in support processes for people with disabilities. Enabling a disabled person to make choices, and to have a life plan not influenced by the entourage, are major challenges. In this context, the aims of the present study are: 1) To design and develop a digital assistant for the elaboration of a life plan that is easily usable by future users (adapted to the cognitive characteristics of people with Down Syndrome (DS); 2) Assess the impact of the use of this assistant on the life plan, autonomy, and quality of life of people with DS. The design process resulted in the development of a mock-up, two prototypes and the final version. Each version has been evaluated (user scenarios to evaluate objective usability and SUS scale and QUEST 2.0 to assess subjective usability and user satisfaction) and improved taking into account the comments from the previous step (37 adults with DS in total). Results confirm the relevance of the improvements made between the different versions and provide a final version offering a high degree of usability. The second objective was to assess the ability of the assistant (final version) to improve the life plan and the autonomy of such people. Evaluations were carried out on 9 adults (5 using the assistant and 4 control subjects who used a computer-based word game, during the same period) using questionnaires relating to the life plan, autonomy, and quality of life of the users. After one month of using the assistant, a positive impact was observed (compared to the control group) on the richness of the life plan and on the person's autonomy using the Ryff psychological well-being scales.< Réduire