Water Efficiency Calculations:
Covering England and Wales

Water efficiency calculations are the Part G calculations used to show that a dwelling’s proposed sanitary fittings and relevant appliances do not exceed the permitted water-use limit. In practical terms, they prove whether the design of the home’s cold and hot water systems complies with the approved water-efficiency standard for that dwelling.

Yes, in the normal domestic Part G sense. In England, Requirement G2 applies only when a dwelling is erected or formed by a material change of use within regulation 5(a) or (b). Wales applies the same basic scope. That means the service is mainly for new-build dwellings and qualifying conversion-created dwellings.

Yes. New dwellings fall directly within the Part G water-efficiency regime. In England, that means complying with the 125 litres/person/day standard unless the optional 110 litres/person/day requirement has been imposed. In Wales, newly erected dwellings are set at 110 litres/person/day.

Yes, where the conversion creates a dwelling that falls within the relevant material-change-of-use route. England’s Approved Document G applies G2 where a dwelling is formed by material change of use under regulation 5(a) or (b), and Wales does the same. In practice, a qualifying conversion still needs a Part G water-use calculation.

The current standard in England is 125 litres per person per day for a new dwelling, unless the optional tighter requirement applies. Where planning permission specifies and conditions the optional requirement, the limit becomes 110 litres per person per day.

In Wales, the current standard is 110 litres per person per day where a dwelling is erected, and 125 litres per person per day where a dwelling is formed by material change of use within regulation 5(a) or (b). That is one of the clearest current differences between England and Wales.

No. In England, 110 litres/person/day is an optional requirement, not the national default. The live national standard is 125 litres/person/day, and the tighter 110 only applies where planning permission specifically requires it as a condition.

Yes. Government planning guidance says local planning authorities can set the tighter 110 litres/person/day standard where there is a clear local need, through local plan policy and the planning process. Regulation 36 then makes that optional requirement bite where planning permission specifically includes it as a condition.

It is the approved calculation methodology set out in Appendix A in England and Annex 2 in Wales. It is used to assess whole-house potable water consumption in new dwellings and qualifying dwelling conversions for compliance against the water-use targets in Regulation 36.

Lock the sanitaryware and relevant white goods down early, check whether the project is working to 125, 110, or the Welsh route, and do the calculation before procurement changes start. The projects that stay on programme are usually the ones that treat Part G as a live compliance item from design stage, not a last-week Building Control notice.