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Subbasement is a floor or set of floors below the main basement floor. In contrast to regular basements where many of them have small amount of vertical space located above ground, subbasements are essentially underground spaces underneath the home or other buildings. Subbasements have similar uses as basements as they're used as storage areas, but least commonly used as living spaces compared to main basements.

During the construction, building a subbasement is more difficult, more costly, and more time-consuming than building a traditional basement as the lowest floor. Plus subbasements are even more susceptible to floods and water damage than the main basement. As a result of all these, subbasements are rare in homes and other buildings and are more often found in dry climates and high elevations.

There are famous landmarks having subbasements. The subbasement in the U.S. Capitol Building is used as storage and that in the White House is used to store guest items. The button in the elevator that take riders to the subbasement when pressed would read "B2" or "SB".