The regulatory requirements for workplace temperatures are set by the Workplace (Health, Safety and Welfare) Regulations 1992. The regulations state that the temperature of indoor workplaces should be "reasonable".
What is the Legal Minimum Working Temperature?
There is no legal minimum working temperature.
However the Approved Code of Practice defines a reasonable indoor temperature as being normally at least 16°C. If the work undertaken is heavy physical work, the temperature should be at least 13°C.
What is the Legal Maximum Working Temperature?
There is no legally set upper limit for indoor working temperatures. This is because factors like humidity and air movement can make a significant difference to thermal comfort levels. The Health and Safety Executive (HSE) recommends that employers should consult with employees or their representatives, to establish ways to cope with high temperatures.
The HSE website has guidance on temperatures in the workplace, as well as information about protecting people working outdoors and protective equipment: HSE Temperature Pages