The definition for overheating contained within table 1.8 of CIBSE Guide A is perhaps the most commonly known definition for overheating within the industry: ‘1% of occupied hours over operative temp. of 28°C’.
This is a simple barometer for defining overheating, but may not be appropriate in all cases. Many occupants, particularly those suited to air conditioned environments, may find temperatures of >24°C inappropriate. In such cases, it may be worth considering the more stringent CIBSE TM52 definition for overheating.
It is always worth establishing what the contractual requirements are for dealing with overheating risk and what is aspirational. It is prudent to agree an appropriate level of thermal comfort with the end user pre-tender, in order to avoid problems at a later date.