Part G Water Efficiency Calculations: A Practical Guide

Part G of the Building Regulations sets out the water efficiency requirements for new dwellings in England, requiring designers to demonstrate that a home will not exceed a maximum wholesome water consumption figure, expressed in litres per person per day. The default national baseline is 125 litres per person per day, but local planning authorities in water-stressed areas can apply a tighter optional requirement of 110 litres per person per day as a planning condition, so the applicable target should always be confirmed against the relevant planning decision notice rather than assumed from the regulations alone.

The calculation itself works by assigning a fitted-value figure to each water-using component in a dwelling, including WCs, taps, showers, baths, washing machines, dishwashers, and any water reuse systems such as rainwater harvesting or greywater recycling, then combining these using the government's water efficiency calculation methodology to arrive at a single per-person daily consumption figure. Where the calculated figure exceeds the applicable target, designers typically need to specify lower-flow taps and showerheads, dual-flush or reduced-volume WCs, or in some cases a rainwater or greywater system, and then re-run the calculation to check the revised specification passes.

Getting this right early in the design process matters because water efficiency compliance is checked at both plan and completion stages, and late changes to fittings or plumbing strategy can be costly to implement once a scheme is designed. For anyone working through these calculations who wants a quick way to check a specification against either the 110 or 125 litre target before committing to it on paper, Kovinto's water efficiency calculator runs the standard government methodology online and lets you test different fitting combinations for free, with a formal certificate available to download once the calculation passes.

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