To deliver effective domestic retrofit at scale, it is essential to understand the current and required capabilities of building practitioners working in the repair, maintenance and improvement (RMI) of existing buildings. Capability research in the construction sector has previously focused on large projects, but small, and particularly, micro-firms that undertake RMI and form 77% of workers in construction, are under-researched. This gap is addressed by the present study on the capabilities of the practitioners and the contextual opportunities to deploy capabilities. The study analysed data from interviews ('n' = 27) with micro-enterprise building practitioners working in the UK’s RMI sector. Template analysis was conducted by applying an established model of behaviour change: Capability, Opportunity, Motivation—Behaviour (COM-B). Under Capability, three main themes were identified: knowledge, business management and individual characteristics. Under Opportunities, the main themes were state action, market and customer demand, technology diffusion, networks and business management. Under Motivation the themes were pride in work, good working relationships, maintaining a viable business and customer satisfaction. Practitioners are continually learning and problem-solving, developing trust and creating positive professional relationships. Working with these existing capabilities, experiential learning on-site and peer-to-peer training are recommended to scale up capability. For capabilities to be deployed, policy must enable opportunities across the multiple contexts micro-enterprise practitioners operate within, including training and incentives across the supply chain network and in stimulating demand. 'Policy and practice relevance' Policy-driven retrofit programmes, such as those providing government funding for retrofit, must work from existing practitioner capability to accelerate capacity and allow delivery at scale. Practitioner knowledge, built over generations, is used to solve problems encountered with existing buildings. Practitioners minimise risk by avoiding unfamiliar technologies and practices. Practitioners develop capability on-site, experientially, so policy must enable practical experience for practitioners. One aspect of capability that needs enhancement to deliver effective retrofit at scale is understanding the building as an integrated system. While learning about individual technologies is important, integrated knowledge of multiple technologies and how they work together is required. To develop industry capability, policy needs to recognise the essential role micro-enterprise practitioners play in delivering retrofit and to harness their existing capabilities in knowledge, problem-solving and business management. Opportunities are needed to develop retrofit capability through peer-to-peer learning, knowledge-sharing between older and younger practitioners, and influential sector networks.