Greywater and rainwater harvesting in Part G calculations
How reused water is counted in the Approved Document G calculation, and when a reuse system is worth considering.
At a glance
Most homes pass Part G with sensible fittings alone. Where the target is tight, or the client wants large baths and generous showers, water reuse can take litres off the result. Appendix A of Approved Document G sets out how to count it.
Greywater recycling
A greywater system collects lightly used water from basins, baths or showers, treats it and supplies it back to WCs or the washing machine. In the calculation:
- The collected volume is based on the fittings feeding the system.
- Only a percentage of that is available for reuse, depending on the system's performance.
- The saving is the smaller of the water available and the demand of the WCs and washing machine it supplies.
Systems should be designed and installed to BS 8525.
Rainwater harvesting
A rainwater system collects run-off from the roof, stores it and supplies WCs or the washing machine. The yield is estimated either from the collection area, a yield coefficient and local annual rainfall, or from a detailed daily figure for the system. It is then shared between the expected occupants (the number of bedrooms plus one, with a minimum of two). As with greywater, the saving cannot exceed the demand it supplies.
Rainwater systems are designed to BS EN 16941-1, which replaced BS 8515.
Is it worth it?
For most single dwellings, reducing the shower flow rate or fitting a lower-flush WC is far cheaper than a reuse system. Reuse makes more sense where:
- the planning condition requires 110 litres and the client wants high-flow showers or large baths;
- the scheme is already installing rainwater harvesting for drainage or sustainability reasons; or
- a larger development wants to show a headline figure well below the limit.
Water softeners
A water softener adds to the result if it uses more than 4% of its incoming water for regeneration. The extra is spread across the occupants. If one is planned, use the manufacturer's regeneration figures in the advanced calculator.
Frequently asked questions
Do I need greywater or rainwater harvesting to pass Part G?
Usually not. Most homes pass with sensible fittings alone. Reuse systems help where the target is tight, for example 110 litres with high-flow showers or large baths.
Which standards apply to water reuse systems?
Greywater systems should be designed to BS 8525 and rainwater harvesting systems to BS EN 16941-1, which replaced BS 8515.
Can a home use greywater and rainwater together?
Yes, but each WC or washing machine can only be counted as supplied by one of the two systems. Our calculator checks this for you.
Model a reuse system
The advanced calculator handles greywater, rainwater and water softeners, alongside full manufacturer specifications.
Open the advanced calculator Free to calculate. No account or payment needed to start.