Part G Water Calculations: What Are They and Do You Need One?

Every new dwelling created by new construction needs to demonstrate compliant water use before Building Control will issue a completion certificate. That requirement sits in Part G of the Building Regulations, and demonstrating it means producing a Part G water calculation – a step that’s simple to complete early but expensive to get wrong once sanitaryware is already fitted.

In this blog you’ll learn:

• What Approved Document G actually covers
• Which factors affect whether your calculation passes or fails
• Whether your project needs a Part G water calculation
• The difference between the 125 and 110 litres-per-person-per-day targets
• How your water calculation affects your SAP score and EPC rating

What is the Part G Water Calculation?

Part G water calculations assess a property’s predicted water use against Approved Document G – the section of the Building Regulations covering sanitation, hot water safety and water efficiency. In practice, “Part G calculation,” “water efficiency calculation” and “water calculation” all refer to the same thing: the water efficiency calculator required under Part G to demonstrate compliant consumption.

Approved Document G sets out six requirements, alongside the water efficiency calculator itself:

• G1 – Cold water supply
• G2 – Water efficiency
• G3 – Hot water supply and systems
• G4 – Sanitary conveniences and washing facilities
• G5 – Bathrooms
• G6 – Food preparation areas
• Water efficiency calculator for new dwellings

The water calculation itself sits under G2, assessing a property’s predicted consumption against the standard 125 litres-per-person-per-day target.

What Factors Affect Your Part G Calculation?

A handful of specific fittings and systems drive whether a property passes or fails its target:

• Flow rate – High flow rates increase water consumption and work against your result. Low-flow fixtures, such as aerated showerheads and tap aerators, bring it back down.
• Toilet flush – Toilets can carry excessive consumption at each flush. A dual-flush system, with a half-flush option for liquid waste, reduces this.
• Bath water capacity – Larger baths increase consumption. Specifying a bath with a lower water capacity helps offset this.
• Appliance consumption – Less efficient washing machines and dishwashers use more water. More efficient appliances bring the figure down.
• Rainwater and greywater harvesting – Not harvesting rainwater or greywater increases the amount of treated water a property consumes. Installing a rainwater and/or greywater harvesting system offsets this.
• Water softeners – Inefficient water softeners increase consumption. Specifying a more efficient model reduces it.

Sourcing accurate flow-rate data for each of these is easier than it used to be. Under the UK’s mandatory water efficiency labelling scheme, introduced under the Environment Act 2021, taps, showers, WCs and other fittings now carry a standardised A–F water label – making it far simpler to confirm verified flow-rate figures at design stage rather than falling back on manufacturer defaults.

Note: Specific details and technical requirements of Part G regulations vary depending on the type of building and its use.

Do You Need a Part G Assessment?

The statutory G2 water efficiency calculator applies to new dwellings created by new construction – that’s a clear-cut requirement before Building Control will sign off the project. Conversions (a material change of use) and extensions sit outside the statutory requirement, unless a specific local planning condition explicitly mandates compliance for that site. If you’re working on a conversion or extension, it’s worth checking your planning conditions before assuming a water calculation isn’t needed.

That’s what our Water Calculations service is built around – working from a project’s actual specified fittings rather than generic assumptions, so the result reflects what’s genuinely being installed, whichever route brought you to needing one.

The 125 and 110 Litres-Per-Person-Per-Day Targets

The baseline Part G target is 125 litres of potable water per person per day. Many local planning authorities apply a stricter optional target of 110 litres per person per day, usually in areas of water stress – so it’s worth confirming which target applies to your specific site before specifying fittings.

Some regions already go further. London developments, for example, must meet the London Plan’s Policy SI 5 target of 105 litres or less per person per day – tighter than the national optional target.

The government has also proposed lowering the national baseline itself, from 125 down to 105 litres per person per day, with the optional target cut from 110 to 100 – aimed at easing water scarcity and unblocking stalled housing developments in constrained areas. This is currently a DEFRA consultation proposal rather than a change already in force, so it’s worth checking the current status against your project’s timeline rather than designing to it as confirmed policy.

Hitting a lower consumption figure than the baseline isn’t just about compliance, either. Some water companies now offer financial incentives for developments that beat the standard targets by a meaningful margin. Our guide to the Environmental Incentives Common Framework covers how those incentive tiers work and what they’re worth per plot.

How Water Calculations Affect Your SAP Score and EPC Rating

The Part G calculation isn’t only a Building Control requirement – the flow rates and fitting specifications it captures feed directly into your SAP calculation. Leave the water calculation until after fittings are already specified, or skip it altogether, and assumed default data gets entered into the SAP assessment instead – data that typically works against you and can drag the final EPC rating down.

What’s Next?

Getting the Part G water calculation right at design stage is straightforward. Fixing it after fittings are already purchased and fitted isn’t – it can mean redesigning a bathroom specification or replacing sanitaryware that’s already in place. Confirming your target and fittings early avoids both.

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We carry out Part G water calculations for self-build, conversion and volume new-build projects, working from your actual fitting specifications rather than generic assumptions.