Part G Water Calculations Explained: Understanding Water Efficiency Rules for New Builds
Part G water calculations help demonstrate whether a new home meets the water-efficiency requirements of UK building regulations. They estimate how much potable water a household could use each day based on the fittings and appliances installed in the property. For developers, architects, building designers and homeowners, understanding these calculations early can make it easier to select suitable products and prepare for building control checks.
In England, Approved Document G covers sanitation, hot water safety and water efficiency. Its water-efficiency guidance includes a calculation method for assessing the potential daily water consumption of new dwellings. The calculation uses product specifications rather than relying only on general estimates of household water use.
This guide explains what Part G water calculations involve, which fittings affect the result, how the assessment works and what to consider when planning a new build.
What Are Part G Water Calculations?
Part G water calculations are assessments used to estimate the potential potable water consumption of a dwelling. They help determine whether the proposed fittings and fixed appliances meet the applicable water-efficiency requirement.
The calculation method considers the water consumption associated with items such as:
- Toilets and their flush volumes.
- Showers and their flow rates.
- Baths and their capacities.
- Washbasin and kitchen taps.
- Washing machines.
- Dishwashers.
- Certain other water-using fittings and appliances covered by the method.
The result is expressed in litres per person per day (l/p/d). This allows the estimated water demand of a dwelling to be compared with the relevant regulatory target.
A Part G calculation is not the same as a plumbing design calculation. It does not establish pipe sizes, water pressure or drainage capacity. Instead, it assesses the potential water consumption associated with the home’s specified fittings and appliances.
It is also important to understand that the result is an estimate based on a standard methodology. Actual household consumption can differ depending on the number of occupants, their habits and how the property is used.
What Is Part G of the Building Regulations?
Part G of the Building Regulations in England addresses several aspects of water and sanitation within buildings. These include:
- Sanitation and washing facilities.
- Hot water supply and safety.
- Water supply to relevant fittings.
- Water efficiency in new dwellings.
Approved Document G provides guidance on ways to demonstrate compliance with the applicable requirements. It is particularly relevant to new residential developments because the water-efficiency assessment can influence the choice of bathroom fittings, kitchen taps and appliances.
The water-efficiency requirement is associated with Regulation 36 and the approved calculation methodology. The relevant target depends on the applicable regulatory provisions and whether an optional requirement has been specified through planning permission.
Although people sometimes use the phrase “Part G calculations” to describe a single water-efficiency assessment, Approved Document G covers more than water consumption alone.
Important: Building regulations differ across the UK. The requirements described here focus on England. Projects in Wales, Scotland and Northern Ireland should be checked against the rules and technical guidance that apply in the relevant nation.
What Is the Water Efficiency Target for New Builds?
For new dwellings in England, Regulation 36 sets a standard water-efficiency target of no more than 125 litres per person per day, calculated using the approved methodology.
An optional requirement of 110 litres per person per day applies where the relevant planning permission specifies that requirement as a condition.
These targets are not simply measurements of the water used by the people living in a house. They are calculated estimates based on specified fittings and appliances.
Standard and Optional Targets
| Requirement | Maximum calculated consumption |
|---|---|
| Standard requirement in England | 125 litres per person per day |
| Optional requirement, where applicable | 110 litres per person per day |
The lower target requires a more water-efficient specification. Developers may need to select lower-flow showers, suitable taps, efficient toilets and appliances that use less water.
Do not automatically assume that every development must meet the 110-litre target. Check the planning conditions and the requirements applicable to the specific site before selecting products or commissioning the calculation.
Planning policies may also introduce additional expectations. These should be reviewed separately from the standard national requirement.
How Does a Part G Water Calculation Work?
The approved methodology estimates water consumption by using information about the fittings and appliances proposed for the dwelling.
Rather than measuring actual water use, the calculation applies standard usage assumptions to product specifications. The individual contributions are then combined to produce an estimated daily consumption figure per person.
Step 1: Collect the Product Specifications
Start by identifying the proposed fittings and fixed appliances for the property.
Useful information includes:
- Shower flow rates in litres per minute.
- Toilet full-flush and reduced-flush volumes.
- Bath capacity.
- Tap flow rates.
- Washing machine water consumption.
- Dishwasher water consumption.
Use accurate manufacturer specifications for the actual products being installed. Where a product has several operating modes or settings, confirm which figures the approved methodology requires.
Step 2: Enter the Relevant Data
The information is entered into a water-efficiency calculator that follows the approved calculation methodology.
The calculation accounts for different types of water use and applies standard factors where required. Some fittings contribute more to the final result than others, depending on their specifications and the methodology.
Step 3: Calculate the Estimated Consumption
The calculator combines the relevant contributions and produces a result in litres per person per day.
This figure represents potential consumption under the standard calculation assumptions. It should not be treated as a prediction of the exact amount of water the future occupants will use.
Step 4: Compare the Result with the Target
The calculated figure is compared with the applicable requirement.
For example, if the relevant target is 125 litres per person per day, the proposed specification must produce a result no higher than 125. If an applicable planning condition requires 110 litres per person per day, the lower limit must be met instead.
Step 5: Review the Specification if Necessary
If the result exceeds the target, review the fittings and appliances contributing to the calculation.
Possible changes include selecting a more efficient shower, reducing tap flow rates, choosing a suitable dual-flush toilet or specifying appliances with lower water consumption.
Once changes have been made, update the calculation using the correct product data.
Which Fittings Have the Biggest Effect on Water Calculations?
The effect of each fitting depends on its specification and the calculation methodology. Several common items deserve particular attention during the design stage.
Showers
Shower flow rate is an important consideration because it influences the amount of water delivered during a shower.
A shower with a lower flow rate may reduce its contribution to the overall calculated consumption. However, product selection should also account for user comfort, water pressure and the requirements of the intended system.
Check the manufacturer’s stated flow rate and ensure it corresponds to the proposed installation.
Toilets
Toilet flush volumes can affect the final result. Dual-flush toilets provide different flush options, while other models may have a single flush volume.
When comparing products, check the full-flush and reduced-flush figures where relevant. Do not rely on a general product description if the calculation requires specific technical data.
Taps
Washbasin and kitchen taps are also included in the assessment. Flow rates can vary significantly between products.
Water-efficient tap specifications may help lower the calculated consumption, but the selected fittings should remain suitable for the intended use and the property’s water supply.
Baths
Bath capacity can contribute to the estimated water demand. Where a bath is included, its relevant specification should be entered correctly into the calculation.
A large bath may affect the result differently from a smaller model, depending on the method and the other fittings installed.
Washing Machines and Dishwashers
Fixed appliances can contribute to the assessment where they fall within the approved methodology. Their water-consumption specifications should be checked carefully.
Using the correct appliance data helps prevent errors caused by entering figures for a different model or an outdated specification.
How Can Developers Achieve a Compliant Result?
The easiest way to manage water efficiency is to consider it early in the design process rather than waiting until the property is nearly complete.
Select Efficient Products Early
Review the available fittings before finalising the bathroom and kitchen specifications. This allows designers to compare flow rates, flush volumes and appliance data before products are ordered.
If the development has an optional water-efficiency requirement, early product selection becomes particularly important.
Use Reliable Manufacturer Information
The calculation depends on the specifications entered. Incorrect flow rates, flush volumes or appliance figures can lead to an inaccurate result.
Keep product datasheets and technical information available so the assessment can be checked against the actual specification.
Assess the Whole Dwelling
Do not focus only on one fitting. A low-flow shower may help, but the final result also depends on toilets, taps, baths and other relevant items.
Reviewing the whole specification provides a clearer picture of where improvements may be needed.
Check Planning Conditions
Confirm the applicable target before deciding which products to use. Where an optional requirement is included in planning permission, it can affect the specification needed to achieve compliance.
Planning conditions and building regulations are related but separate considerations. Both should be reviewed as part of the project.
Keep the Calculation Updated
If a specified product changes during construction, check whether the replacement has different water-consumption figures.
An updated product specification may affect the calculated result. Maintaining accurate records helps the project team demonstrate how the final design meets the applicable requirement.
Common Mistakes in Part G Water Calculations
Errors can occur when the calculation is prepared using incomplete or inaccurate information. Common issues include:
- Using generic figures instead of the actual manufacturer’s specifications.
- Entering the wrong shower flow rate.
- Confusing full-flush and reduced-flush toilet volumes.
- Using data for a product that has since been replaced.
- Assuming the standard target applies when planning conditions require a lower figure.
- Treating estimated consumption as actual measured household use.
- Confusing water-efficiency calculations with plumbing or drainage design.
- Failing to check which national building regulations apply to the site.
Checking the product data and the applicable target before finalising the assessment can help prevent avoidable revisions.
When Should a Part G Water Calculation Be Completed?
Ideally, the water-efficiency calculation should be considered during the design and specification stage of a new dwelling.
At this point, the project team can compare different fittings and appliances before purchasing decisions are finalised. If the calculation identifies a problem, there is more opportunity to make adjustments without disrupting the construction programme.
The assessment may also need to be reviewed if the specification changes later. The precise documentation and submission arrangements should be confirmed with the relevant building control body or project professional.
For larger developments, preparing calculations at an appropriate stage for each dwelling type can help maintain consistency. However, the result should reflect the actual relevant specification rather than assuming that every property is identical.
Frequently Asked Questions
What is a Part G water calculation?
It is an assessment that estimates the potential potable water consumption of a dwelling using an approved methodology and product specifications. It helps demonstrate compliance with the applicable water-efficiency requirement.
What is the standard Part G water-efficiency target in England?
The standard target for new dwellings in England is a maximum calculated consumption of 125 litres per person per day. An optional 110-litre target applies where the relevant planning permission specifies it as a condition.
Is Part G water efficiency the same as a plumbing calculation?
No. The water-efficiency calculation estimates potential consumption from relevant fittings and appliances. Plumbing design calculations address different issues, such as pipe sizing, flow requirements and water supply arrangements.
Which products are included in the calculation?
The methodology includes relevant fittings and fixed appliances, such as showers, toilets, taps, baths, washing machines and dishwashers. The exact data required depends on the approved method and the products installed.
Can changing a shower help achieve compliance?
It can. A shower with a lower specified flow rate may reduce its contribution to the calculated water consumption. The overall result still depends on the rest of the dwelling’s fittings and appliances.
Does a Part G calculation show how much water residents will actually use?
No. It provides a standardised estimate based on product data and assumed usage patterns. Actual consumption can differ because of household size, behaviour and other factors.
Do the same Part G rules apply throughout the UK?
No. Building regulations differ between England, Wales, Scotland and Northern Ireland. Always check the applicable national requirements and guidance for the project location.
Final Thoughts
Part G water calculations help demonstrate that a new dwelling meets the relevant water-efficiency requirement. By using accurate product specifications and an approved calculation method, developers and designers can estimate potential daily consumption and identify where more efficient fittings may be needed.
The key steps are to confirm the applicable target, collect reliable manufacturer data, assess the whole dwelling and update the calculation when the specification changes. Addressing water efficiency early can make compliance easier to manage and support more responsible water use in new homes.
