[Cob] cob project - footing

[Carol Bixler]

I am planning on building a patio surround and corner oven/bbq on an approximately 15' x 15' cave top that is made out of extremely hard old cement/concrete.  I want to build a free form bench and wall around two sides, with the oven in the corner.  Can I just start out on the cement, or do I need extra footing.  This is my first project, so any suggestions or advice is welcome and appreciated.   

 Carol