HUBZone Expansion Act of 2015 This bill amends the Small Business Act to expand the HUBZone (historically underutilized business zone) program of Small Business Administration loans to small businesses in HUBZone areas for communities affected by base realignment and closure. The expansion includes the areas that can be used by small businesses whose principal offices are located in base closure HUBZones, with at least 35% of their employees residing in: a HUBZone, a census tract in which the base closure HUBZone is wholly contained, a census tract that intersects the boundaries of the base closure HUBZone, or a census tract contiguous with these census tracts. HUBZone eligibility assistance for base closure areas is extended from five years to eight years after their closure.