Brilliant idea for collecting rainwater to use in a house, and I will just let the design execution speak for itself. More pictures on the architect's site here.
That thing in the living room is a rainwater storage tank. Water is so central to our lives, and Paul Morgan Architects make it central to this house. "Within the living room the ceiling wraps down to an internal water tank. The tank cools the ambient air temperature of the living room during summer, supplies rain water, and structurally carries the roof load. Excess water drains to an external tank, and is used for flushing toilets, irrigating the garden, washing wetsuits [it's near the coast in Victoria, Australia] and occasionally for drinking."