In April 2010 the UK Government updated Part G of the Building Regulations with new requirements for water efficiency. This specifies that all new dwellings need to be designed, in order to achieve a projected water consumption less than 125litres/person/day (or much lower for Code for Sustainable Homes dwellings).

In order to prove compliance against the water performance targets in Building Regulations 17K and the Code for Sustainable Homes. There is a specific methodology that needs to be followed for assessing potable water consumption in new dwellings.

The water calculation incorporates: the measurement of tap flow-rates, bathtub sizes and toilet flush-volumes, as well as rainwater and greywater systems.

Assessments under Part G Building Regulations do not apply to extensions, but may apply to conversions or renovations where all the plumbing is being replaced. Some Local Councils may also apply tighter water consumption figures according to the Local Planning conditions.