The shelter designs in this booklet were developed primarily for use in new houses, but some can be used in existing houses. When a new house is being built, the builder/contractor can construct walls, foundations, and other parts of the house as required to accommodate the shelter. Modifying the walls or foundation of an existing house as necessary for the construction of a shelter is more difficult. As a result, some of the shelter designs in this booklet are not practical for existing houses. The following sections discuss this issue further. In this booklet, the term “retrofit” refers to the process of making changes to an existing house.