Stoves that can sit on a 12mm hearth
Why are many stoves allowed to sit on just 12 millimetre of hearth? Because they DO NOT heat the surface underneath them to more than one hundred degrees Celsius. If a stove does NOT have this status it must have 125mm (5") of non-combustible material underneath it. Many stoves (especially 8kW or under) do have this status nowadays.
https://www.stovefitterswarehouse.co.uk/pages/hearth-regulations