A concrete slab foundation consists of a layer of concrete which is, in many cases, poured directly on grade over a substrate of gravel. This is typically the least expensive type of foundation for home construction, and it is an effective option for small home construction in warmer climates. It can be problematic in cold climates, however, where frost heaving can cause the slab to move and crack.
Alternatives to a simple slab-on-grade foundation can help to protect the slab in cold climates. A frost-protected foundation has insulation to trap heat from inside the building around the edge of the foundation and prevent the ground beneath it from freezing. A T-shaped foundation has poured concrete footers that extend below the frost line to help stabilize the slab.
When you're building a house on a slab foundation, it's relatively difficult to install plumbing, wiring or other mechanical systems on the ground floor, since there's no space under the floor through which these systems can be run. The concrete slab also serves as the subfloor, and it's generally less comfortable underfoot than the wood subfloor used on a raised foundation.
A crawl-space foundation consists of poured concrete footers that extend below the frost line around the perimeter of the home. The concrete perimeter extends above grade level, typically a minimum of 18 inches, and a wooden sill plate and floor joists rest on top of the concrete. The space between the grade and floor structure provides access to the area under the floor for mechanical systems, and it also protects the structure from moisture and cold.
Crawl-space foundations are more expensive to build than slab foundations, but they are less expensive than a foundation on a full basement.
A full-basement foundation is the most costly of the commonly built home foundations. It requires that the space under the house be excavated to the depth of a full-height basement, followed by the construction of footers and perimeter foundation walls of either poured concrete or concrete blocks. It has the advantage of adding living or utility space to the home and providing easy access to mechanical systems.
In areas where soil conditions result in instability or poor drainage, or where bedrock is close to the surface, full-basement foundations may not be practical or cost effective. Full basements are more common in the Northeast and Midwest in the United States, and much less so in the South and the West.