masonry.
If you are building it anywhere in CA you will require a permit.
For a permit you will require plans with a stamped signed set showing the footing calculations and design.
As you already probably know that it will have to be a gas fired fire place as wood is not legal. A licensed plumber is required to do the gas line and hook -up.
All this info would be supplied in your set of plans and submitted to the building and planning dept. to obtain your permit .