If your stove is NOT going into a chimney recess and is certified not to heat the hearth underneath it to more than 100 degrees centigrade (often they are raised higher, sometimes with log stores underneath) then it can have a simple minimum 12mm thick non-combustible hearth superimposed hearth with NO constructional hearth underneath it.
The minimum hearth size is 84cm x 84cm. There must be 150mm minimum between the stove and the edge of the hearth – this increased to minimum 300mm in front of the stove (actually 225mm for stoves designed to be used with the door closed but 300mm is best practice).
Note: If your stove is not one that is certified regarding overheating the hearth then a constructional hearth will be required. Details can be found within HEARTH>key building regs.