The statutory social work role in England and Wales regularly involves assessment and ‘best interests’ decision making under the Mental Capacity Act (MCA) 2005. Legal frameworks that permit substitute decision making have, however, been challenged as a contravention of the human rights of people with disabilities. This article aims to answer the question: ‘Can mental capacity be theorised for social work assessment in England and Wales in a way that is compatible with social work ethics and values?’ Findings are that mental capacity is a complex and disputed concept best described using a constructivist ‘social model’ which situates it as a property of an individual’s relationships. Such a model suggests that assessments are a joint endeavour and that the quality of the relationship between assessor and assessed is crucial. The present legal framework disincentivises but does not prevent capacity assessment based on a social model. The existing literature suggests that further research might identify ways in which social work capacity assessment could enhance rather than diminish the autonomy of the people involved. These findings have implications for social work research, education and post qualification training.