The hearth (or more accurately said "hearth extension") is the area in front of your fireplace that basically protects the wood portion of your home from the fire in he fireplace. But how big should your hearth be?
Well, it generally depends on the type, age, and size of your fireplace. Different areas and different years had different regulations - so "one size fits all" does not usually work. But a s a general rule, the hearth distance from your fireplace should be about half the width of your fire box. So for instance, if you take a measuring tape and your fireplace is 30 inches wide, the n your hearth should extend at least 15 inches away from the fireplace.
Devereaux Van Dyne, Certified Professional Home Inspector