Part G water efficiency explained: 125 vs 110 litres
Every new home in England is designed to use a limited amount of water. Most people never hear about it until building control asks for the figure.
What Part G sets
Part G of the Building Regulations covers sanitation, hot water safety and water efficiency. The water efficiency part, requirement G2 and regulation 36, sets a limit on how much water a new dwelling is designed to use: 125 litres per person per day (Approved Document G).
It's a design figure, not a meter reading. It's worked out from the fittings specified: the WCs, taps, showers, baths and appliances. How the occupants actually live doesn't come into it.
Where 110 litres comes from
There's an optional, tighter requirement of 110 litres per person per day. It only applies when a planning permission includes a condition requiring it. This is common in areas with water stress, including much of the south and east of England.
The only way to know is to read the planning decision notice. If there's a condition about water efficiency or "110 litres", the tighter figure applies. If not, it's 125.
What's included in the figure
Both limits include a fixed 5 litres per person per day for outdoor use, such as watering gardens. So the indoor fittings really have about 120 litres, or 105 under the tighter limit.
Who it applies to
The requirement applies to new dwellings, including those created by a change of use, such as an office converted to flats. It doesn't apply to extensions or refurbishments of existing homes.
Two ways to show compliance
- The fittings approach. If every fitting is within the limits in Table 2.1 (125) or Table 2.2 (110) of Approved Document G, that's enough. No calculation needed.
- The full calculation. If anything is over those limits, or there's no shower, or there's a waste disposal unit, water softener or water reuse system, a calculation using the national methodology is needed.
There's more on choosing between them in fittings approach vs full calculation.
When it's checked
The calculation is usually submitted with the building control application and confirmed at completion. Building control needs to be told the dwelling's water efficiency figure, so it's worth settling the bathroom and kitchen specification early. Swapping a shower or WC after it's been ordered is far more painful than adjusting it on paper.
Run the numbers
The free calculator checks a dwelling against 125 or 110 litres, shows where the water goes, and finds the smallest change that passes.