An expandable holding tank includes a base supporting a first sidewall to define a liquid retaining enclosure. A second sidewall travels up and down one of an exterior face or an interior face of the first sidewall from a stored position to an operative position. In the stored position, the second sidewall is nested with the first sidewall to position a lower edge of the second sidewall adjacent to the base. In the operative position, the second sidewall is telescopically extended relative to the first sidewall to place the lower edge of the second sidewall at a distance from the base and an upper edge of the second sidewall extending above an upper edge of the first sidewall.