This paper provides insight into the participatory development process of a self-report questionnaire for adolescents: the Best Interest of the Child Self-report questionnaire (BIC-S). The BIC-S is based on the 'Best Interest of the Child' model. This model articulates, in line with the UN Convention on the Rights of the Child, 14 childcare conditions promoting the wellbeing and development of young people. The primary intention of the BIC-S is to give young people in care a voice regarding decisions in legal and care areas that impact their future. The development process of the BIC-S consisted of three main phases (exploration, consultation, pilot), containing a total of five different research stages. Adolescents placed in secure residential care, care professionals and university students were involved in the process. We developed a differentiated instrument that enables young people to express their own views on key aspects of their current and future living conditions. The digitized questionnaire is unique in its comprehensiveness, accessibility and attractiveness for young people, and generates an evaluative profile of care conditions from the adolescents themselves. The paper shows how a theoretically sound questionnaire can be constructed to conform to standards set by adolescents themselves. The BIC-S can serve as a productive vehicle for assessment and shared decision-making in the field of youth care - both at the micro level of individual treatment and at the meso level of evaluating group programmes. Further research into its use in practice is recommended.