Meeting the UK’s net-zero greenhouse gases target by 2050 requires transdisciplinary engineering, it requires efficient exchange and collaboration between engineering and social science, between engineers and policy makers within the national government. Based on ethnographic fieldwork conducted within the UK’s department for Business, Energy and Industrial Strategy (BEIS), this paper explores how technical and policy expertise were mobilized and combined in a recent change in utility-scale solar policy. Taking a model developed by BEIS’ engineering advice team in collaboration with the established renewable policy team, this paper looks at what it means to give and receive engineering advice in the context of utility-scale solar regulation. Looking at the model design process from both the engineer’s and policy advisor’s perspectives highlights how concepts of expertise, disciplinarity compatibility and opposition impact policy and outcomes. The modelling process was successful in helping the negotiation and reconciliation of technical and social concerns to enable a change in utility-scale solar regulation satisfactory to industry and constituents. By drawing on this case, this paper ends on a wider discussion about how the generation of mutual trust and development of interactional knowledge between engineers and policy advisers enables TE in policy practice.