Constructing the foundation for a home addition requires more planning than the original foundation for the house. The proper planning of the foundation determines the level of the floor of the addition. In many cases, it is desirable for the floor level of the addition to be equal to the floor level of the existing house. A small error in the planning and construction of the addition foundation can leave floor levels mismatched.
Calculate the depth of the foundation. Begin with the intended floor level of the house. This may require a cutting or drilling a hole in the exterior wall of the existing house to determine the floor level. From there, calculate the height of the footings, foundation, plate, joists and flooring to determine the necessary depth of the excavation. Stake out the perimeter of the intended addition.
Excavate the soil from the foundation area. Contract with professional with a backhoe or excavator for larger projects or start digging with a shovel for smaller areas. Excavate to the level of the calculated bottom of the footings creating trenches for the foundation allowing about a two foot work space on each side of the foundation. If you're using excavation equipment, it is common to shovel the foot or so of dirt closest to the house's existing foundation by hand to reduce the chance of damage to the home.
Build the footings. Use 2-by-6-inch or 2-by-8-inch lumber to create forms for the footings. Place the form lumber 12 inches apart and stake in place. Level the top of the footings at the calculated position for the top of the footings. Nail the footing forms to the stakes. Pour the cement into the forms and level the cement with the top of the forms by pulling a board across the top of the forms. Trowel the cement lightly and allow to dry.
Add the foundation materials. Build the foundation of either cement blocks or permanently treated lumber, depending on the owners preference. Concrete blocks are laid in place with mortar between the footings and first course of blocks. Continue adding courses of blocks until the calculated top of the foundation has been reached. Treated lumber foundations are built with a bottom sill, a horizontal board bolted to the footing using anchor bolts, studs and a top sill. Studs are vertical support members of the foundation spaced every 16 inches on center. The top sill is nailed to the top of the studs. The 1/2-inch treated plywood covers the sides of the wood foundation using eight penny nails.
Back fill the work areas on each side of the foundation by shoveling the dirt into place. Earth moving equipment can be used for larger projects. If possible, place any subsoil or clay at the bottom of the back fill and place topsoil at the top of the excavation. Save some soil to fill any settling that will occur.