The icon for a basement is a house with a window and a door and under it has a rectangle smaller than the one used for the shape of the house and inside it is a stairs to represent the stairs use to go down to a basement.
This is a Basement icon. We've made it in iOS style
, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 13,900 icons, all in the same style and quality.