PowerSwitch

Could houses not be designed with a "shelf" and rainwater storage tank(s) high up at the "ends" of the building to collect water from the gutter and store it above the top-floor water cistern level - and combine that with manual levers (or automated system) within the house so that water can be taken from the storage tank(s) and from the mains if the tank(s) are empty?

And then, could a manual lever direct the waste bath, sink, washing machine and so on water to the drains or external storage (for use in the garden) as the lever-controller deems appropriate?

Good idea. We were only talking today about a neighbour's house which as a bedroom with three outside walls and is a devil to keep warm.

If you built a square-ish, two-storey house, each upper storey corner room could contain an insulated stormwater tank. Bedrooms would thus only have one outside wall.

Hmmm.

Yes, that benefit as well!

Just buy a redundant Water Board header tank, convert it to collect the water that falls on it and build a house underneath it.

Seeemple!

Water in useful amounts is very heavy, considerable structural work would be required to store say 10 tons of it a height. And 10 tons is very little by household standards.
10 tons of water can be purchased for about £20 from the water company, suggesting a very lengthy payback time on the expense of building work.
A tank at or below ground level might be better idea, despite the need for a pump.

Another possibilty is a tank on a garage roof, filled from the house roof.
It is relatively easy to construct a garage to withstand say 10 or 20 tons on the roof. Water from such a tank could be used for watering the garden, and for flushing a downstairs toilet.
I know someone who has done this, though at the present low price of water it should be considered more of a disaster prep, than a money saving idea.