Chimney inspection and research essay

I’m In the attached galley picture is an example of when the hearth require extension. When the fire place opening is 6 feet square or greater the extension from the face should be 20 inches and 12 inches from the side. If the fire place opening is less than 6 feet square the hearth only has to be 16 inches from the front and 8 inches to the side. However, the 3 door from combustible still applies from the top.