The urgent transition to a zero-carbon economy requires building professionals to be supportive of, and prepared for, delivering zero-carbon buildings. Building professionals are important ‘middle actors’ who can either enable or inhibit such societal transitions. This paper explores building professionals’ perspectives on delivering zero-carbon buildings, leading to a practical synthesis of knowledge and skill requirements and training pathways. It draws on the middle-out perspective (MOP) and secondary analysis of three UK case studies. The MOP suggests that middle actors in a system are not perfectly responsive to policy push or market pull. Instead, they exert their own agency and capacity downstream to customers and clients, sideways to other middle actors and, occasionally, upstream to policy-makers. The data comprise: interviews and a small survey with building professionals on energy efficiency and refurbishment; the observation of a specific commercial office building design and development and a workshop to identify zero-carbon knowledge and skill needs of middle actors. Building professionals addressed in this paper include vocational trades, engineers, designers, project managers and ‘clerks of works’ (site-based quality technicians). Although formal training pathways for these roles differ, each can develop expertise ‘sideways’ interacting between professions.