My Grandparents house had a water tank out side, this was above the outside toilet area I think, so a brick extension, flat roof, trhe water tank was on top of that. Could hold a lot of water and was ideal for watering plants etc during hot weather I guess.
Things like this are now seen as old fashioned but if houses were built like this we could all harvest a lot of rain water (esp given how much we are getting at the moment).
Sometimes the old ways of the past had good solutions.