The water calculator in a sustainability statement

The water efficiency calculation planning authorities ask for is not a planning invention. It comes from Part G of the Building Regulations, which sets a baseline of 125 litres per person per day for new dwellings and provides an optional tighter requirement of 110 litres per person per day that only applies where a planning condition imposes it. The local plan policy is the trigger. The calculation and the compliance route sit in the Building Regulations.

So the sustainability statement has two jobs here. Show that the scheme can meet the tighter figure with a realistic fittings specification, and accept the condition that will secure it.

Where the tighter standard comes from

Part G requirement G2 limits the estimated consumption of wholesome water per person per day in a new dwelling. The baseline figure of 125 litres includes an allowance of five litres for outdoor use, so the internal budget is 120 litres.

The optional requirement of 110 litres works the same way: 105 litres internal plus the five litre external allowance. It is not a free-standing planning standard and it cannot be imposed by a policy alone. The mechanism is that a local plan policy justifies applying the optional requirement, and a condition on the planning permission then engages it so that building control enforces the lower figure.

That sequence has two consequences worth knowing. If there is no condition, the baseline applies whatever the local plan says. And where a policy asks for a figure below 110 litres, it is asking for something the optional requirement does not provide, so the mechanism has to be different: a planning obligation, or a bespoke condition, or in some areas a water neutrality requirement with its own arithmetic.

Check the adopted policy wording, and check whether the authority has published a standard condition. It saves an argument at decision stage.

How the calculation works

Approved Document G gives two routes, and most submissions use the calculator.

The water efficiency calculator for new dwellings works from the installed fittings. Each fitting has a consumption figure, expressed as litres per use, litres per minute or litres per cycle, which is multiplied by a fixed use factor representing typical frequency of use. The results are added, divided by the assumed occupancy, and the five litre external allowance is added at the end.

The alternative is the fittings approach, a table in the Approved Document giving a set of maximum fitting specifications that together are deemed to achieve the standard. It is simpler and less flexible. Where a scheme wants a generous bath or a powerful shower, the calculator is the only way to show that the rest of the specification compensates.

Run the calculation per dwelling type rather than once for the scheme, and use the fittings actually specified. A calculation based on placeholder values that the eventual specification does not match is a discharge problem later, not a planning one.

Fittings that move the result

Four items decide almost every outcome.

Shower flow rate is the largest single variable. The difference between an unrestricted mixer shower and a flow-regulated head is several litres per person per day on its own, and a scheme that fails at 110 litres can usually be brought back by the shower alone.

Bath capacity is second, measured to the overflow rather than to the brim. A deep freestanding bath is expensive in this calculation, and where a client wants one, something else has to give.

WC flush volume comes third. Dual flush cisterns are assessed on an effective flush volume derived from the full and reduced volumes and their assumed frequencies, so a four and two and a half litre dual flush performs very differently from a six and four.

Taps are fourth, and basin taps matter more than kitchen taps because of the use factors applied. Flow regulators and aerators are the low-cost fix here.

White goods are included where they are supplied as part of the dwelling. Where they are not, the calculator applies default values, and specifying efficient appliances you do not supply gains nothing.

Rainwater and greywater in the calculation

The calculator allows a reduction where a rainwater harvesting or greywater reuse system supplies specified uses, and this is where authorities look hardest.

The reason for the scrutiny is that these systems depend on maintenance by the occupier over decades. A system that is bypassed after a pump failure in year three delivers nothing, and the dwelling reverts to whatever the fittings alone achieve. Several authorities therefore prefer to see the standard met on fittings, with the harvesting system as an additional benefit rather than a necessary part of the calculation.

If you do rely on a system, show the yield calculation, the storage volume, the uses served, the mains backup arrangement and the maintenance regime. And be clear which document holds what: rainwater harvesting also appears in drainage submissions, usually with a much smaller attenuation credit than applicants hope for, and the two claims should not contradict each other. Keep the hydraulic figures consistent with the Drainage Strategy.

Greywater systems carry additional design and regulatory considerations around backflow prevention and water quality, so they need a specialist rather than a catalogue selection.

Evidencing compliance at planning stage

What the case officer needs is short and specific. A Sustainability Statement section setting out the standard being met, the route used, and a completed calculation per dwelling type, with a fittings schedule attached.

The fittings schedule should list, for each item, the type, the consumption figure and its units, and the source of that figure. Manufacturer data is fine. An assumption is not, unless it is labelled as one.

Three things commonly go wrong. A calculation run at the baseline of 125 litres when the policy requires 110. A single calculation covering a scheme with four dwelling types of different configurations. And a result at exactly 110.0 litres, which leaves no tolerance for a specification substitution during construction. Aim a few litres below the target for the same reason you design eaves below the limit.

Where the site has a capacity or supply question as well as an efficiency question, that is a different document. A Water Supply Assessment deals with whether the network can serve the development, which the efficiency calculation does not address.

The condition that secures it

Expect a condition requiring the dwellings to be constructed to achieve no more than 110 litres per person per day calculated in accordance with the methodology in Approved Document G, with evidence submitted before occupation.

Read the wording for two things. Whether it names the calculator or the fittings approach, since being locked into one route removes flexibility. And what evidence it requires, because a condition asking for the as-built calculation is straightforward while one asking for metered consumption after occupation is neither precise nor enforceable against a future owner.

Discharge is usually simple: the as-built calculation, the final fittings schedule, and confirmation from building control. Keep the fittings schedule updated through construction, because the discharge application is where a substituted shower head surfaces.

Interaction with water neutrality policies

In a small number of areas the requirement goes well beyond the optional standard, and the arithmetic is different.

Where a water neutrality requirement applies, as it does in the Sussex North water supply zone following advice from Natural England, new development has to demonstrate that total water use in the area does not increase. That is a catchment-level test, not a per-dwelling one, so meeting 110 litres is a starting point rather than an answer. Schemes there typically target a lower per-person figure and then secure offsetting through retrofit of existing properties or another agreed mechanism.

Separately, some plans in water-stressed areas apply a figure below 110 litres through policy, which needs a mechanism other than the optional requirement, and a few apply an efficiency standard to non-residential development through a certification route instead.

Do not confuse any of this with nutrient neutrality, which addresses nitrogen and phosphorus loading on protected habitats and has nothing to do with the water efficiency calculation. The two appear together in some catchments and are answered by entirely different evidence.

1. Check whether the adopted policy applies the 110 litre optional requirement, a lower figure, or a neutrality test.
2. Run the calculator per dwelling type using specified fittings.
3. Aim a few litres below the target for tolerance.
4. Attach the fittings schedule with sources.
5. Keep the schedule live through construction for the discharge application.

Enter your postcode on PlanScope to see which sustainability and water documents your scheme is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.