Belowstairs is a term used to describe the lower level of a building, particularly in the context of large houses or estates. Some alternative synonyms for belowstairs include basement, cellar, lower level, subterranean, underground, and lower floor. These words are often used interchangeably to describe spaces that are located below the ground level. Additionally, subterranean and underground may refer to spaces that are completely below ground. In certain contexts, belowdecks, lower decks, or bilge can also be used as synonyms for belowstairs, particularly on ships or boats. Overall, these synonyms can be useful when describing locations or spaces that are located below ground level.