Regs are somewhat complicated but boil down to:
Freestanding stove should be on 84cmx84cm slab of min 5" thick (or 12mm thick if stove allows for 12mm hearth as many do - see product description)
Stove in a recess should be on a hearth that extends out of the recess by minimum 50cm and extends left and right from the recess by 15cm. Slab of min 5" thick (or 12mm thick in some cases).
In both of above situations hearth should protrude in front of glass by min 22.5cm (30cm is good practice) and some stove manuals may demand more (especially if stove has a large glass).
Read the article above for my best attempts at explaining the full regs. Probably the hardest article I wrote for The Stovefitter's Manual (Julian).