Per capita consumption in a water supply assessment

Part G of the Building Regulations sets a water efficiency standard for new dwellings of 125 litres per person per day, calculated using the water efficiency calculator for new dwellings. Where a local planning authority has adopted the optional requirement, the figure tightens to 110 litres per person per day, and that optional standard is widely applied across water stressed areas of England. Both figures include a fixed allowance of 5 litres per person per day for outdoor use, so the internal budget is 120 or 105 litres respectively.

Everything else in a water supply assessment follows from which of those two numbers applies to your site.

Why consumption is the headline number

The calculation is deterministic. The water efficiency calculator takes the flow rate or capacity of each fitting type, applies fixed use factors, and produces a litres per person per day figure for the dwelling.

That makes it checkable, which is why officers go straight to it. A fittings schedule and a calculator output are the whole of the evidence, and a statement that the dwelling will meet the standard without the schedule attached does nothing.

Because the calculation uses fixed use factors rather than measured behaviour, the only variables you control are the fittings. There is no credit for occupant behaviour, metering or education.

The optional tighter standard and where it applies

The 110 litre figure is an optional requirement under Part G, and it only bites where the local plan has adopted it and where the authority imposes it as a condition. The planning practice guidance sets out that an authority must justify the optional standard with evidence of water stress, and must apply it through a local plan policy rather than ad hoc.

The areas of serious water stress are identified by the Environment Agency, and most of the south and east of England falls within them. Check the adopted local plan policy rather than assuming, because adoption varies even within a water stressed area.

In London, the London Plan requires the 105 litre internal figure, which is the 110 litre standard less the outdoor allowance, and it applies across all boroughs.

Where the optional standard applies, the condition should refer to the Building Regulations requirement and the figure. A condition requiring a lower figure than 110 litres per person per day is not supported by the Part G framework, since the optional requirement is a single defined standard. Some authorities nonetheless seek a lower figure through policy, and that is a point to test at application stage.

Using the water efficiency calculator

The calculator is published by the department responsible for building regulations and takes a defined set of inputs.

Where an appliance is not fitted by the developer, the calculator uses a default value, which is usually worse than a specified efficient appliance. On a tight scheme, specifying the white goods is sometimes what closes the gap.

The bath is the single largest contributor in most calculations. A 230 litre capacity bath against a 170 litre one moves the result by several litres per person per day on its own, and in apartment schemes a shower-only bathroom with no bath makes the standard easy to hit.

Fittings schedules that achieve the target

A workable schedule for the 110 litre standard typically looks like this.

1. Dual flush WC at 4 litres full and 2.6 litres part, with a realistic flush ratio.
2. Shower at 8 litres per minute, flow regulated.
3. Bath at no more than 170 litres to overflow.
4. Basin taps at 4 litres per minute or less.
5. Kitchen tap at 6 litres per minute.
6. Washing machine at 8.17 litres per kilogram or better where specified.
7. Dishwasher at 1.25 litres per place setting or better where specified.

Flow regulation is the detail that fails on site. A tap rated at 4 litres per minute delivers that at a stated pressure, and in a high pressure system it delivers more. Specify flow regulators or pressure-reducing valves and say so in the submission, because a verification check measures what comes out of the tap.

Do not specify a water softener if you can avoid it, since the calculator penalises it.

Rainwater and greywater contributions

Rainwater harvesting and greywater recycling can be counted in the calculation, and in practice they are rarely the efficient way to reach the standard in a dwelling.

A rainwater system needs a tank, a pump, a separate distribution network with marked pipework, backflow protection and a mains top-up. The capital and maintenance cost per litre saved is high compared with a better shower head, and the maintenance obligation falls on the occupier who did not choose it.

Where they do make sense is in larger schemes with a significant non-potable demand, such as landscape irrigation or communal cleaning, or where a site has a constrained mains supply and reducing peak demand has independent value.

If a system is proposed, the yield calculation has to be shown, with the roof area, the runoff coefficient, the rainfall data source and the demand profile. A Water Supply Assessment carrying that arithmetic is checkable; one asserting a percentage saving is not.

Non-domestic water demand

Part G's per capita calculation is for dwellings. Non-domestic buildings are assessed differently, usually through BREEAM water credits or a local plan policy expressed as a percentage improvement over a baseline.

For a mixed-use scheme, run the residential calculation under Part G and the commercial element under the relevant standard, then present the two separately. Combining them produces a figure that means nothing.

Commercial demand also matters for the supply capacity question, which is distinct from efficiency. The water undertaker's capacity response deals with whether there is enough water and pressure at the connection point, and a Utilities Assessment brings that together with the other services. Where foul flows are the issue instead, that belongs in a Foul Sewage Assessment.

Conditioning and verifying the standard

The usual condition requires the dwellings to be constructed to achieve the stated litres per person per day, with evidence submitted before occupation.

Evidence means the completed water efficiency calculator output for each dwelling type, the fittings schedule as installed, and the manufacturer data for each item. Collect it during construction rather than afterwards, because tracing what was actually fitted in plot 14 eighteen months later is slow work.

Where a substitution happens on site, rerun the calculator before accepting it. A shower head swapped from 8 to 10 litres per minute for availability reasons can take a dwelling over the standard, and the contractor making the swap will not know that.

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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.