Clacton St. John's primary school in Essex, UK, resulted from a development exercise which studied different design options. Built last year, the school is now being rigorously monitored for thermal performance. Peter Page, Assistant County Architect, and Brian King, Environmental Engineer in the Architects' Department, Essex County Council, here explain the step-by-step evaluation process which led to a design with a selective solar passive approach combined with a groundwater-to-water heat pump for underfloor heating, with controlled ventilation. The design is expected to halve the applied energy requirements for a primary school compared to the maximum values in the current DES design note.