Part G Water Efficiency Guide
Part G Water Efficiency: Everything You Need to Know
Part G of the Building Regulations sets out requirements for water efficiency in new dwellings, aiming to reduce the amount of water consumed by each household. A water efficiency calculation is a legal requirement for all new build dwellings and conversions, demonstrating that the property’s water consumption will not exceed the permitted daily limit.
With water resources under increasing pressure across the UK — particularly in the south and east of England — Part G water efficiency compliance is an essential part of the building regulations process for every residential project.
What Is the Water Efficiency Target?
Part G specifies that a new dwelling must not use more than 125 litres per person per day. This is the standard national target that applies to all new residential buildings in England and Wales.
However, some local planning authorities enforce stricter targets:
- 110 litres per person per day — commonly required by London planning authorities where London Plan compliance is needed.
- Other local targets — some local authorities outside London have adopted their own water efficiency targets through local plan policies, particularly in areas of water stress.
If you are unsure which target applies to your project, UKBC will check with your local planning authority.
What Is Assessed in a Water Efficiency Calculation?
The calculation considers the water consumption of all fittings and appliances that use water within the dwelling:
Fittings Assessed
| Fitting | What Is Measured |
|---|---|
| Toilets | Flush volume (litres per flush) |
| Baths | Capacity to overflow (litres) |
| Showers | Flow rate (litres per minute) |
| Wash basin taps | Flow rate (litres per minute) |
| Kitchen sink taps | Flow rate (litres per minute) |
| Dishwashers | Water consumption per cycle (litres) |
| Washing machines | Water consumption per cycle (litres) |
The water efficiency calculation uses standardised assumptions about how frequently each fitting is used per person per day, combined with the actual flow rates and capacities of the specified products.
Maximum Fittings Consumption
Part G specifies maximum fittings consumption levels for the 125 litres per person per day target:
- WC — 6/4 litre dual-flush (or 4.5 litre single flush)
- Bath — maximum 170 litres capacity to overflow
- Shower — maximum 8 litres per minute
- Wash basin taps — maximum 6 litres per minute
- Kitchen sink taps — maximum 8 litres per minute
Selecting fittings that meet or exceed these standards is the simplest route to compliance.
Factors That Affect the Water Calculation
The results of your water efficiency calculation are influenced by several factors:
Flow Rates
The flow rates of taps and showers are the most significant variable. High-flow power showers and mixer taps can push consumption well above the target. Specifying water-efficient fittings from the outset is essential.
Bath Capacity
Large, deep baths consume significantly more water per use than standard baths. If the dwelling includes a bath, its capacity to the overflow must be considered. Some luxury or freestanding baths have capacities that make compliance very difficult.
Toilet Flush Volume
Modern dual-flush toilets (typically 6/4 or 4/2.6 litres) perform well in the calculation. Older-style single-flush toilets using 9 or 13 litres per flush would fail.
Water-Saving Technologies
Additional technologies that can improve the water calculation result include:
- Rainwater harvesting systems — collect and store rainwater for non-potable uses such as toilet flushing and garden watering.
- Greywater recycling — treat and reuse water from showers and wash basins.
- Water softeners — can reduce water consumption in hard water areas by improving the efficiency of soap and detergent use.
- Aerated taps and showers — reduce flow rates while maintaining the perception of adequate flow.
When Should You Get a Water Efficiency Calculation?
The ideal time to commission a water efficiency calculation is at design stage. This allows you to:
- Confirm that your specified fittings will meet the target before you purchase them.
- Make changes to fitting specifications if needed, while options are still flexible.
- Speed up building control sign-off by having the calculation ready before completion.
Leaving the calculation until late in the project risks discovering that the installed fittings do not meet the target — requiring costly replacements.
What Information Does UKBC Need?
To carry out a water efficiency calculation, UKBC requires:
- Complete floor plans indicating the number and location of all wet rooms (kitchens, utility rooms, bathrooms, en-suites, cloakrooms)
- The number of each type of fitting (showers, baths, WCs, wash basins, kitchen sinks, dishwashers, washing machines)
- Specific product details if you have already selected fittings (manufacturer, model, flow rate, capacity)
- Confirmation of whether the local authority requires the 110 or 125 litres target
If you have not yet selected specific fittings, UKBC can advise on suitable products that will meet your target.
Water Efficiency and Other Regulations
Water efficiency calculations interact with several other aspects of building compliance:
- SAP calculations — hot water consumption affects the energy calculation in SAP. More efficient hot water fittings reduce energy demand for water heating, potentially improving the SAP result.
- The London Plan — developments in Greater London typically need the stricter 110 litres target, which is demonstrated through the water efficiency calculation as part of the London Plan compliance package.
- Part G hot water safety — Part G also covers hot water safety, including requirements for limiting stored hot water temperatures to prevent scalding.
- Waste water heat recovery — WWHR systems, which recover heat from shower wastewater, can be included in both the water calculation and the SAP calculation.
Water Efficiency Calculations With UKBC
UK Building Compliance provides fast, accurate water efficiency calculations for new build dwellings and conversions across the UK. We aim to complete most calculations within 24 hours of receiving the necessary information.
Our service includes:
- Full Part G water efficiency calculation and formal compliance report
- Advice on fitting selection to meet your target
- Identification of the cheapest and most practical changes if the initial calculation exceeds the limit
- Coordination with SAP calculations and other compliance services
We offer water efficiency calculations as part of our complete compliance packages, alongside SAP calculations, air tightness testing, and EPC production.
Call 01455 242297 or request a free quote today.
Frequently Asked Questions
What is the Part G water efficiency target?
Part G requires that new dwellings must not exceed 125 litres of water per person per day. Some planning authorities, particularly in London, enforce a stricter target of 110 litres per person per day.
Do I need a water efficiency calculation for a new build?
Yes. Water efficiency calculations are a legal requirement under Part G of the Building Regulations for all new build dwellings and conversions. The calculation must demonstrate that water consumption stays within the permitted limit.
What fittings are assessed in a water efficiency calculation?
The calculation assesses toilets, baths, showers, wash basin taps, kitchen sink taps, dishwashers, and washing machines. The flow rates, flush volumes, and capacities of these fittings determine the overall water consumption figure.
How long does a water efficiency calculation take?
UKBC aims to complete most water efficiency calculations within 24 hours of receiving the necessary information. You will receive a formal report demonstrating compliance or advising on changes needed.
What happens if my water calculation exceeds the target?
If the calculation shows consumption exceeds the target, UKBC will advise on which fittings to change. Common solutions include selecting lower-flow taps and showers, dual-flush toilets, or smaller-capacity baths.
Ready to get started?
Get your compliance sorted today — call us or request a free quote.