The optional water efficiency standard and how it applies

The baseline for a new home in England is 125 litres per person per day under requirement G2 of the Building Regulations. The optional tighter standard is 110 litres per person per day, and it only applies to your build if the local planning authority imposes it by condition, which it can only do if its adopted local plan contains a policy justifying the requirement. So the answer to whether you have to hit 110 is not found in the Building Regulations. It is found in your decision notice and in the local plan behind it.

Both figures include a fixed allowance for external water use, so the internal target is five litres per person per day lower than the headline number in each case.

Two numbers, and where each comes from

Requirement G2 of Schedule 1 to the Building Regulations sets the standard for water efficiency in new dwellings. The optional requirement was introduced as part of the set of optional technical standards, alongside the access standards and the nationally described space standard, so that authorities could apply a tighter figure where local circumstances justify it.

The mechanism is deliberately awkward. An authority cannot simply prefer the tighter standard. It has to have a policy in the adopted local plan, that policy has to be justified by evidence, and it then imposes the standard through a condition on the planning permission. Building control enforces compliance once the condition is in place.

That means three documents have to line up: the local plan policy, the planning condition, and the building control submission. Self-builders regularly meet the standard at building control without realising the condition exists, and then struggle to discharge it because the evidence was never formatted for the planning file.

The evidence an authority needs to impose it

National guidance expects an authority to consider the existing water efficiency of its housing stock, the need for water efficiency in the area based on water resources evidence, and the impact of the requirement on viability. Areas classified as being under serious water stress are where the case is easiest to make, and many authorities in the south and east of England have adopted the standard on that basis.

If you are applying in an area where the condition appears but the local plan predates the optional standards, it is worth asking on what policy basis the condition is imposed. A condition has to be necessary, relevant to planning, relevant to the development, enforceable, precise and reasonable, and one imposed without a policy foundation is open to challenge under section 73 of the Town and Country Planning Act 1990.

That is a narrow argument, and for most self-builders meeting 110 costs less than fighting it. The specification changes involved are modest, and they are decisions you would want to make anyway on a house you intend to live in and pay the metered bills for.

Calculator or fittings table

Approved Document G offers two routes to demonstrate compliance. The water efficiency calculator works from the specification of each fitting, its flow rate or capacity, and standardised usage assumptions, and produces a consumption figure per person per day. The simplified fittings table lists maximum values for each fitting type as a deemed to satisfy route.

The fittings table is provided as a route to the 125 litre baseline. Where the tighter optional figure applies, the calculation route is the one to use, and you should check the current Approved Document because the routes and the calculator have been revised over time.

Practically, the calculator is better anyway. It lets you trade a generous bath against efficient taps and a low flush WC, rather than forcing every fitting to a maximum.

Hitting 110 without making the house unpleasant

The specification decisions that matter, roughly in order of effect:

• WC flush volume, since the WC is the largest single internal use in most homes, and dual flush units with a low full flush do most of the work
• shower flow rate, where the difference between a generous rain head and a well specified efficient head is large
• bath capacity, measured to overflow, which the calculator treats as a volume rather than a usage rate
• basin and kitchen tap flow rates, controlled by aerators and flow regulators
• appliance consumption, where washing machine and dishwasher figures are taken from the fitted appliances

Rainwater harvesting and greywater recycling can be counted in the calculation, subject to the rules in the Approved Document, but they are rarely the efficient way to close a small gap on a single dwelling. Fittings are. A harvesting system also brings maintenance, pump energy and a tank that has to go somewhere, none of which appears in the compliance figure.

The complaint you hear most is about showers. A low flow head with poor design feels weak. A well designed one at the same flow rate does not, and it is worth specifying a product you have stood under rather than one that appears on a compliance list.

The condition, and discharging it

A typical condition requires the dwelling to be constructed to achieve a maximum water consumption of 110 litres per person per day calculated in accordance with the methodology in the Building Regulations, with evidence submitted to the authority before occupation.

To discharge it, submit the completed calculation with the schedule of fittings as installed, not as specified at design stage. If the shower was changed on site, the calculation has to reflect the change. Keep the product data sheets, because the flow rates on them are what the calculation relies on.

Where the scheme is also subject to a Sustainability Statement or an energy condition, deal with all of them in one submission with a covering schedule. Authorities charge per discharge application, and separate submissions for each condition cost more and take longer.

Where this meets water neutrality

The optional standard is a per dwelling efficiency requirement. Water neutrality is a catchment level requirement that development must not increase total water abstraction, applied in specific areas following advice from the environmental regulators, and it is a much heavier obligation involving offsetting through retrofit of existing properties or other measures.

If your site is in a water neutrality area, expect the 110 litre standard to be the starting point rather than the answer, with additional measures required and a Water Supply Assessment forming part of the application. The two regimes are separate and both can apply.

Illustrative example: a self-build detached house in a serious water stress area receives permission with a 110 litre condition. The fittings schedule specifies a 4 and 2.6 litre dual flush WC, an 8 litre per minute shower, 5 litre per minute basin taps and a 170 litre bath, and the calculator returns a figure below the requirement with a small margin. The margin matters, because a change of bath on site can consume it. Illustrative only, and any real specification needs to be run through the current calculator.

Leave that margin deliberately. A calculation that lands exactly on 110.0 will fail the moment somebody swaps a tap.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which documents your plot is likely to need.