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What Is the Meaning of Subbasement?

Skip Davis

A subbasement is a floor or level beneath the main basement of a residential or commercial building. Subbasements serve a variety of purposes, depending on whether they are in a public building or home. Like other building floors, the subbasement has advantages and disadvantages.

A couple is standing out front of their house under construction.

If you want to build a house with a subbasement, make sure you live in an appropriate climate.


The most common purpose for residential subbasements is storage space. Subbasements also provide a cool, isolated area for installing the house's boilers and furnaces. One example of this is the Biltmore House in Asheville, North Carolina. The Biltmore House, one of the largest houses in the world, used a subbasement as a storage space and for boiler installation. For commercial buildings, a subbasement may be used as a parking garage or, in some hotels, a swimming pool area.

Proper Climate

To build a subbasement, you must live in an area with a dry climate. Subbasements are susceptible to water damage in humid regions such as the Southeast. In cold climates, subbasements must receive the same type of insulation as basements. According to the U.S. Department of Energy, a properly insulated basement reduces energy costs for the entire household; the same is true for insulated subbasements. However, the Department of Energy also says the exterior insulation of basements and subbasements is susceptible to insect damage.

Moisture Control

According to the National Institute of Building Sciences, poorly built basements and subbasements are generally accepted in many modern buildings. As a result, subbasements are often subject to water leaks. The Institute adds that the success of a subbasement's design is gauged by how well the subbasement controls moisture. To ensure the highest quality of moisture control, the design of the subbasement's waterproofing system must match the subbasement's overall structural design.

Famous Subbasements

Some of America's iconic landmarks have subbasements. In Washington D.C., the U.S. Capitol building and the White House have subbasements. The subbasement in the Capitol building was built as a storage space, while the White House's subbasement serves as a dressing area for guest performers. New York City's major train hub, Grand Central Station, has a subbasement. Known as M42, Grand Central's subbasement was used to store the building's electrical converters; the existence of the subbasement was kept secret during World War II to prevent power outages caused by enemy attacks.