The present invention provides a support frame assembly for a firebox for a built-in gas barbecue grill assembly having a work structure. The support frame assembly has a first end substantially adjacent the firebox and a second end substantially adjacent a floor supporting the built-in grill assembly. In one embodiment the support assembly has a first mating member for supporting the firebox, and a second mating member for mating with the work structure to connect the support frame assembly to the work structure.