In an existing house, removing part of the slab and replacing it with a thickened section would involve extensive effort and disruption inside the house. Therefore, building a shelter with concrete or concrete masonry walls in an existing slab-on-grade house is generally not practical. You can, however, build a wood frame shelter, because its walls are not as heavy and do not require the support of a thickened slab. A wood frame shelter can be created from an existing room, such as a bathroom or closet, or built as a new room in an open area in the house, such as a garage.

You can also build a shelter as an addition to the outside of a slab-on-grade house. This type of shelter must have not only proper footings but also a watertight roof. Because a shelter built as an outside addition will be more susceptible to the impact of missiles, it should not be built of wood framing. Instead, it should be built of concrete or concrete masonry. Access to this type of shelter can be provided through an existing door or window in an exterior wall of the house.