Abstract: A prefabricated fireplace foundation in which the fireplace's floor is installed directly on top of a structurally rigid foundation box. The foundation box incorporates an intake air chamber which opens into the fireplace through use of an exhaust air stack that projects through the fireplace's floor from the box's top wall, and that opens into the outside atmosphere at an intake port in the box's sidewall. A valve is provided to open and close the intake port as desired by the user, the valve being operable through use of a handle accessible at the front of the fireplace. The foundation box also incorporates an ashes chamber which opens into the fireplace through use of an ashes trap that projects through the fireplace's floor from the box's top wall. The ashes trap allows fireplace ashes to be discharged into the box's ashes chamber, an ash door being provided by which the ashes can be withdrawn when cleanout is required.