Keep reading to find out the seven biggest differences between two popular foundation types-slab vs. While both foundation types are used extensively, each one is best suited to specific situations and conditions. Crawl space foundations usually require additional support walls beneath the interior of the house as well. A crawl space foundation features exterior concrete walls that elevate the bottom of the house 18 inches or higher above grade (ground level), providing an accessible area to reach plumbing and wiring should it need maintenance in the future. As the term implies, a slab foundation is a thick concrete slab on top of which a house is built.