Raindrops falling on the roofs of new buildings in one south London borough will be recycled in future.
Climate changes could increase demand on water supplies
Southwark Council has agreed that all new developments need to include rain catching devices to preserve water.
About 40,000 litres of rain falls on the average London roof each year, which can be re-used on site.
Environmentalist Donnachadh McCarthy said the move will save energy used to pump water around and cut the cost of buying water from suppliers.
Around 50% of water used in the home does not need to be of drinkable quality.
Rain harvesters can therefore be used to supply water for flushing toilets, washing machines, watering plants and gardens and other general cleaning tasks.
Mr McCarthy, who lobbied the council to make the changes, said he was delighted with the news and hopes other councils will follow suit.
He said: "This rainwater revolution is long overdue.
"Over 40,000 litres of rain fall on the average London roof each year, yet we still insist on flushing precious drinking water that has been pumped for miles, down our loos.
"This new requirement will not only save the environment but also reduce costs as all new buildings are water-metered."
Southwark Councillor Richard Thomas told BBC News Online that climate change and London's growing population would increase demand for water.
"It is absurd to use drinking quality water for flushing the toilet and watering the garden when there are cheap and easy to use alternatives to recycle rainwater, making Southwark cleaner and greener," he said.