The Grosvenor Estate is also experimenting with rainwater harvesters. This idea is that rainwater can be collected in a central tank and then used to flush lavatories and supply any so-called 'greywater' requirements.

Mains cold water is then used only for drinking or when the tank is dry, something which could happen during summer months.

At Smithy Farm on the Eaton Estate, a 400 litre tank was installed below ground in a field to the rear of the buildings. The rainwater is collected at a central point on each building and then piped to a control unit housed within one of the plant rooms. When required, water from the tank is pumped back to the buildings.

The first re-charges at Smithy Farm Buildings indicate that on average in excess of 60% of the gross water consumed in each year by each commercial occupier is from the harvested supply. As the cost of water increases, the payback periods on these systems will naturally reduce; the system also minimises the development's reliance on the mains water supply – a  hidden benefit to the local community.