However, the hearth extension material can be just 3/8 inch thick if the bottom edge of the firebox opening is at least 8 inches above the hearth extension. Under some circumstances the minimum depth of a hearth plate is 12mm (see below). Also, the hearth extension must extend at least 16″ from the front of the fireplace and 8″ on the sides for smaller fireplaces. A hearth extension must be at least 2 inches thick. hearth measured perpendicular to the plane of the fireplace opening shall be increased by not less than a) 50 mm for an elevation above 150 mm and not more than 300 mm, and
ft. or bigger the front extension needs to be increased to at least 20" and the side extensions to at least 12" beyond the fireplace …
Hearth dimensions: A fireplace hearth should extend at least 16" past the front edge of the fireplace and at least 8" beyond each side of the fireplace opening. Where the fireplace opening is 6 sq. When the opening of the fireplace is at least 6 square feet, the hearth extension needs to extend at least 20″ from the front of the fireplace, and 12″ on the sides.
This is illustrated in the diagram below, courtesy of the fine folks at A hearth must extend a minimum of 300mm (12 inches) in front of the fire or stove and 150 mm (6 inches) to the side of the fire or stove (see diagram 4). According to www.Rumford.com, building codes require a fireplace hearth to be at least 4 inches thick.