An expandable secondary condensate drain pan comprises a drainage basin formed by sidewalls that extend upwardly around a perimeter of the drain pan from a base. An expansion section allows portions of the base and sidewalls to be expanded or compressed. When located along a side of the drain pan, the base of the expansion section may be stepped such that the base at the side of the pan is higher than the base toward the center of the pan, allowing drainage to cascade away from the side of the pan. The expansion section may be strengthened against cracking by a relatively thin coating or film. A method for installing a pre-manufactured expandable secondary condensate drain pan under an air conditioning unit comprises adjusting the size of the drain pan, placing the drain pan on a support, and setting an air conditioning unit on the drain pan.