The Collaborative International Dictionary
Understairs
Understairs \Un"der*stairs`\, n. The basement or cellar.
Wiktionary
understairs
n. A basement or cellar constructed, or accessed from underneath the stairs
Usage examples of "understairs".
As she folded the ironing board away, and put it in the understairs cupboard, she wondered how she would feel about returning to Buckingham Palace.
She dragged the vacuum cleaner free from its entanglement with the ironing board in the understairs cupboard.
Gillian caught her breath then and pulled Jensy close within the shadows of the small understairs pantry.
You could call it whatever you wanted-the lord of dark places, the emperor of understairs, every kid's worst nightmare.