If you have houses spontaneously burning down, that's a problem.
International Code Council
Residential Code for One and Two Family Dwellings
2009
Online version, be informed.
http://publicecodes.cyberregs.com/icod/ ... /index.htm
I mostly use Chapter 3.
You may want to check out Chapter 10.
The first page, item "C" with 20" Hearth extension, see illustration below, Figure R1001.1 FIREPLACE AND CHIMNEY DETAILS



FIX IT!