Where the building services include mechanical ventilation with no heating or cooling element this is considered “free-running”. These buildings do not need to follow the air-conditioned building criteria.
If the building services do not include any mechanical cooling, then the risk of overheating is down to the use of the building fabric to afford attenuation of the diurnal swings / solar gains. Therefore the adaptive approach to thermal comfort from CIBSE AM10 is the correct approach to define and assess summer overheating /thermal comfort against. Following the Adaptive model is seen as allowing a lot of tolerance for the required indoor conditions.
A full DTM assessment will be required to show the predicted summer internal conditions.