Building a shelter inside a house on a crawlspace foundation is more difficult than building a shelter inside a house on a basement or slab-on-grade foundation. The main reason is that the entire shelter, including its floor, must be separate from the framing of the house. A shelter built inside a house cannot use the floor of the house. The shelter must have a separate concrete slab floor installed on top of earth fill and must be supported by concrete or concrete masonry foundation walls. An alternative approach, which may be more economical, is to build an exterior shelter on a slab-on-grade adjacent to an outside wall of the house and provide access through a door installed in that wall.

Ventilation in the area below the floor of the house is also an important issue. The wood framed floor of a house is also an important issue. The wood framed floor of a house on a crawlspace foundation is typically held 18 to 30 inches above the ground by the foundation walls. The space below the floor is designed to allow air flow through so that the floor framing will not become too damp. It is important that the installation of the shelter not block this air flow.