To begin the construction of the pizza oven foundation you first need to establish a solid flat surface. If building directly on soil this will simply mean removing the grass with a spade to expose the soil. If you are to build on an existing structure you will need to ensure that the weight of the pizza oven foundation and the pizza oven itself will not cause any problems. As the materials used may be extremely heavy it is likely that if an oven were to be built on a wood platform that cracks or other failures may occur.
Once a suitable surface has been prepared you may begin construction of the foundation. Material used may include brick, besser blocks, rock or wood such as recycled railway sleepers. The materials may be laid directly on bare soil and an initial concrete foundation is not required. Construct a four walled box as the foundation up to the height you would like the hearth of the pizza oven to sit. Ideally you want the hearth to be high enough that viewing within the oven does not require extensive bending, however not so high that placing of pizzas or cleaning is difficult. Once the box is complete you may fill in the center of the box with soil, sand or another readily available material up to about 20cm from the surface. Upon this newly created cube you will want to place a small concrete foundation to provide a solid surface for the sand and then the fire bricks. You can either create your own concrete from sand, cement and aggregate (small rocks) or purchase premixed concrete in bags. Fill up the cube with concrete 5cm from the surface. You want to leave 5cm spare to insert builders sand to provide a completely flat surface for the fire bricks. The sand is critical to create a flat surface and to provide insulation to prevent heat from dissipating below the oven.