The total volume of this corner store building can be calculated by finding the volume of each rectangular prism and adding them together.
Volume of horizontal prism = length x width x height
Volume = 30m x 6m x 9m = 1,620 m3
Volume of smaller perpendicular prism = length x width x height
Volume = 21m x 6m x 9m = 1,134 m3
Total volume = 1,620 m3 + 1,134 m3 = 2,754 m3
Therefore, the total volume of this corner store building is 2,754 cubic meters.