NAHB analysis of the Survey of Construction (SOC) shows that the market share of new single-family homes built on slabs has been generally increasing, whereas homes with basements or crawl spaces have been on the decline since the year 2000.
56% of all new single-family started in 2014 were built on slab foundations, 28% had a full/partial basement, and 15% selected a crawl space. The remaining 1% included raised supports, earthen, and other foundation types.
The foundation types for residential construction are closely related to climatic conditions, especially the frost line. Building codes normally require foundations to be built at or below frost depth. In colder areas, where the frost depth may be several feet below the ground, houses are predominately constructed with full or partial basements. The division with the highest share of full/partial basements is the West North Central (83%), followed by New England (78%), the East North Central (68%), and the Middle Atlantic (64%). However, the share of homes with a full/partial basement is low in the Southern and the Pacific divisions.
The use of slab foundations is more popular in the West South Central (96%) and South Atlantic (67%) divisions, where the climate is warm and/or clay soil is expansive. Slab foundations are quicker and cheaper to construct, compare to full/partial basements and crawl spaces. However, the adoption of slab foundations is declining and was surpassed by crawl spaces in 2014 in the East South Central division, which is one of the three divisions in the South Region.
Slab foundations are commonly used in the West region, including the Pacific and Mountain divisions. But the large variation in geography and climates leads to different trends of foundation types in these two divisions.
In the Pacific division, new single-family homes with full/partial basements are uncommon, with an average of 3.3% from 2003 to 2014. Slab foundations are most common in the Pacific division, followed by crawl spaces. In contrast, new single-family homes with full/partial basements in the Mountain division increased to almost 50% by 2014, whereas homes with slab foundations declined from 54%, the highest percentage in 2005, to around 39% recently.