Water efficiency standards and the optional requirement

New homes in England are built to a water efficiency standard of 125 litres per person per day under Part G of the Building Regulations. Where a council has an adopted local plan policy justified by local evidence of water stress, it can apply the tighter optional requirement of 110 litres per person per day through a planning condition, and building control then enforces it. That is the whole mechanism, and the detail that trips people up is that the tighter figure cannot be met using the simple fittings table.

The optional requirement is a ceiling as well as a floor. A council cannot use this route to demand a standard tighter than 110 litres per person per day, and national guidance is clear that local policies should not set standards above those the optional requirement provides.

The baseline and optional standards

Part G requirement G2 limits the estimated consumption of wholesome water per person per day in a new dwelling. The baseline applies everywhere and needs no policy to bring it into effect.

Both figures include a fixed allowance for external water use, so the internal consumption a designer has to achieve is lower than the headline number suggests. That allowance is the reason a scheme that looks comfortably compliant on a fittings calculation sometimes is not.

Non-residential development sits outside this regime entirely. Water efficiency in offices, schools and retail is dealt with through assessment schemes and through local plan sustainability policies, which is where a BREEAM Pre-Assessment picks it up.

How the tighter standard is applied by a council

Three things must be in place, and if any one is missing the requirement cannot be imposed.

1. An adopted local plan policy seeking the tighter standard, supported by evidence of local water stress and an assessment of the effect on housing viability.
2. A condition on the planning permission requiring the dwellings to meet the optional requirement.
3. A building control submission demonstrating compliance through the prescribed calculation.

The planning condition is the link. Without it, building control has no basis for applying anything other than the baseline, whatever the local plan says. So if you are an applicant and the condition is worded loosely, ask for it to be tightened rather than left ambiguous, because a vague condition creates an argument at discharge stage.

Where a council seeks a standard tighter than the optional requirement, for example through a policy aiming at a lower figure, that part of the policy cannot be delivered through Part G. It can only be achieved voluntarily or through a separate mechanism, and an applicant is entitled to say so.

The fittings approach and the calculator

Approved Document G sets out two routes to demonstrate compliance, and they are not interchangeable.

The fittings approach is a table of maximum consumption values for each fitting. Specify within every value in the table and the dwelling is deemed to meet the baseline. It is quick, it needs no arithmetic, and it only works for the baseline figure.

The water efficiency calculator is the methodology that produces an estimated litres per person per day figure from the specified fittings, using fixed use factors and standard assumptions for appliances. The tighter optional requirement has to be demonstrated this way. In practice that means someone has to run the calculation for each house type and keep the output with the building control file.

Run the calculation at design stage rather than at handover. Discovering that the specified bath and shower combination misses the target after the bathrooms are ordered is an expensive way to learn the arithmetic.

Design consequences for taps, showers and cisterns

Meeting the tighter figure is achievable with ordinary products, but it constrains the specification in ways that matter to a house builder.

• Shower flow rate is the largest single variable, and a high flow shower will consume most of the available allowance on its own.
• Dual flush cisterns at the lower end of the available range, rather than the default, become necessary in most calculations.
• Basin and kitchen tap flow rates need specifying with flow regulators rather than left to the supplier.
• Bath capacity to overflow is an input, so a large freestanding bath has a real cost in the calculation.
• Appliance values are fixed in the methodology, so an efficient washing machine does not help.

That last point catches people out repeatedly. The calculation uses standard figures for appliances the occupier will buy, so the only levers are the fittings installed as part of the building.

Where rainwater or greywater reuse is proposed, the methodology allows a deduction, but the systems have to be specified, installed and maintained. Councils will usually condition maintenance arrangements before accepting a reuse deduction, and the practical consequence is worth weighing against simply specifying tighter fittings.

Water stressed areas and local evidence

The Environment Agency classifies areas of England by water stress, and that classification is the evidence base most councils rely on when justifying the tighter policy. In seriously water stressed areas the policy is close to standard, and applicants should expect the condition.

Water companies' resource management plans provide the supporting local picture, including the supply demand balance and the headroom available for growth. Where a scheme is large enough that supply capacity is a question in its own right, a Water Supply Assessment addresses the connection and the network, which is a different question from per-person efficiency.

Government has consulted on tightening the baseline standard as part of wider building standards reform. Check the current position in Approved Document G before assuming the figures above are still the operative ones.

Condition wording and discharge

A workable condition names the standard, names the calculation method and names the stage at which evidence is provided. A condition that simply requires "water efficiency measures" is unenforceable and helps nobody.

Discharge normally runs through building control rather than through a separate planning application, because the compliance evidence is the Part G calculation. Some councils nonetheless attach a planning condition requiring the calculation to be submitted to them, which duplicates the building control process. Where that happens, submit the same document to both and say so.

Keep the water efficiency commitment consistent with the rest of the sustainability evidence. A Sustainability Statement claiming a lower figure than the calculation supports is the kind of inconsistency an officer will notice and query.

Interaction with water neutrality requirements

Water neutrality is a different thing again and the two get conflated. Efficiency policy asks each new dwelling to consume less. Neutrality, which applies in specific catchments where abstraction affects a protected habitat, asks that total demand across the area does not increase at all as a result of new development.

Where neutrality applies, meeting 110 litres per person per day is a starting point rather than an answer. The remaining demand has to be offset, usually by retrofitting efficiency measures in existing buildings or by a credit scheme, and the offsetting has to be secured before permission is granted. Those schemes are catchment specific and the requirements change, so confirm the current position with the authority and the relevant statutory body rather than relying on a general description.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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