Water Efficiency Calculator for New Dwellings

Although the final result is straightforward, the calculation itself includes several fixed-use assumptions and fitting-specific formulas. It is therefore worth completing it before sanitaryware and appliances are ordered, rather than discovering near completion that the selected specification exceeds the required target.

What does a new-dwelling water efficiency calculator do?

The calculator assesses the expected use of wholesome water within an individual dwelling. It does not measure actual household behaviour and it is not based on a future occupant’s water meter readings.

Instead, it uses the performance of the proposed water-using fittings and fixed appliances. These commonly include:

• WCs and their full and reduced flush volumes
• Bathroom and cloakroom basin taps
• Kitchen and utility-room taps
• Showers and their flow rates
• Baths and their capacity
• Dishwashers
• Washing machines
• Waste-disposal units
• Water softeners
• Rainwater harvesting systems
• Greywater recycling systems

The relevant consumption figures are combined with the fixed-use factors within the water efficiency methodology. The final result is then compared with the target that applies to the project.

What is the water efficiency target for a new dwelling?

For most new dwellings in England, the standard water efficiency target is:

A tighter optional requirement of:

may apply where it has been imposed through the planning process. The lower target does not automatically apply to every development, so the planning decision notice and conditions should be checked before the calculation is finalised.

Some local planning policies and water-company incentive schemes may also refer to more demanding targets. Where that happens, confirm exactly which target and calculation method the relevant organisation expects before relying on the result.

The wider regulatory requirements can be found in Approved Document G on GOV.UK .

Which projects count as new dwellings?

The requirement is primarily concerned with newly created homes rather than routine plumbing work in an existing property. Typical projects include:

• New-build houses
• New-build flats
• Blocks containing several new dwellings
• Housing developments with repeated house types
• Conversions that create one or more new homes
• Commercial-to-residential conversion projects
• Buildings subdivided into separate flats

An ordinary bathroom refurbishment, replacement kitchen or extension to an existing home will not normally require a new whole-dwelling calculation unless the work creates a separate dwelling or Building Control has specifically requested one.

There are some important differences between new-builds, conversions and work to existing homes. Our separate guide explains when a Part G water efficiency calculation is required in more detail.

What information should you gather first?

The calculation is only as reliable as the specification entered. Manufacturer data sheets, sanitaryware schedules and appliance specifications should be used wherever possible.

Provisional figures can be used during the early design stage, but the calculation should be updated once the final products are selected. The report submitted for completion should reflect the specification that is actually going to be installed.

How the calculation works in practice

Each fitting contributes to the overall predicted consumption. A high-flow shower, large bath or inefficient WC can increase the result, while lower-flow fittings and efficient appliances can reduce it.

The important point is that the dwelling is assessed as a complete specification. A single efficient tap does not guarantee a pass, and one higher-consumption fitting does not necessarily cause a failure. The combined result is what matters.

1. Select the applicable target. Confirm whether the dwelling is being checked against 125 or 110 litres per person per day, or another clearly defined local requirement.
2. Enter the number of fittings. Record how many WCs, taps, showers, baths and appliances are included within the dwelling.
3. Confirm their performance. Enter the relevant flow rates, flush volumes, capacities and appliance-consumption figures.
4. Add any relevant water systems. Include water softeners, waste-disposal units, rainwater harvesting or greywater recycling where applicable.
5. Review the result. Compare the calculated litres-per-person-per-day figure with the selected target.
6. Adjust the specification if necessary. Change the fittings that are contributing most to the total and recalculate before products are ordered.

What happens if the dwelling fails?

A failed result does not necessarily mean the entire design must change. It usually means that one or more parts of the water-fitting specification need to be reviewed.

Common adjustments include:

• Reducing shower flow rates
• Choosing more efficient basin or kitchen taps
• Using lower-volume dual-flush WCs
• Selecting a bath with a lower capacity
• Reviewing washing-machine or dishwasher values
• Correcting provisional figures that do not match the final products

It is generally better to improve the actual fitting specification than to enter optimistic figures simply to obtain a pass. Building Control may ask for product information, and the calculation should remain consistent with the fittings installed on site.

One calculation per plot or one per house type?

The water efficiency requirement applies to each dwelling, but developments often contain several plots built to exactly the same house type and specification.

Where the number of bedrooms, fittings, appliances and water systems are identical, the same underlying calculation can be associated with each relevant plot. For example:

Each plot should still be clearly identified within the project records. If a plot later receives a different shower, bath, WC or appliance specification, its calculation should be updated accordingly.

When should the calculation be completed?

The best time is during design and specification, once realistic fitting information is available but before everything has been ordered.

Completing it early allows the design team to:

• Check whether the proposed fittings meet the target
• Compare alternative products before ordering
• Identify high-consumption fittings
• Meet a planning condition before work progresses too far
• Avoid delays when Building Control requests the final evidence

The calculation should be reviewed again if the specification changes during construction. Substituting a shower, bath or WC can alter the result, even where the replacement product appears broadly similar.

What should the final report contain?

A useful water efficiency report should provide more than a pass statement. It should allow someone reviewing the project to understand which dwelling was assessed, which target was used and how the final figure was reached.

A clear report should normally record:

• The development address
• The house or flat type
• The associated plot numbers
• The selected litres-per-person-per-day target
• The fittings and appliance values entered
• Any rainwater or greywater assumptions
• The complete calculation breakdown
• The final predicted water consumption
• Whether the dwelling meets the selected target
• The date on which the calculation was prepared

This is commonly described as a Part G water efficiency certificate, although it is not a certificate issued or approved by the government. It is a calculation record prepared as evidence for the project and Building Control.

Online calculator, spreadsheet or consultant?

A straightforward new dwelling can often be calculated online where the specification is known. A spreadsheet can also be used, but the person preparing it must apply the methodology and fixed-use factors correctly.

Professional assistance may be sensible where the project has unusual water-reuse systems, uncertain planning requirements, numerous differing specifications or disagreement over how the methodology should be applied.

The price also varies considerably between completing the calculation online and appointing someone to prepare it. See our comparison of Part G water efficiency report costs for typical options and multi-plot examples.

Complete the calculation online

Easy Compliance Tools guides you through the details needed for each house or flat type and displays the calculated water use as you work.

You can check the result and see whether the specification meets the selected target before paying. When the calculation is ready, you can download a structured PDF report containing the project details, fitting breakdown and final result.

The calculator also supports repeated house types and plot ranges, making it suitable for a single self-build, a block of flats or a larger residential development.

Start the water efficiency calculator

Frequently asked questions