25000 liters is equivalent to over 6500 gallons, which is more water than my entire home consumes in a month and weighs about 50,000 pounds - or, if you prefer, 25,000 kilograms. However you look at it, it's a lot of weight to have over your head, and I'm pretty sure that NO residence will ever keep that much water overhead.
No home I have ever lived in had an overhead water tank or, indeed, an attic mounted water heater. No home I have ever worked on had such. No home I have ever visited.
I think it isn't nearly as essential as you claim.