Water efficiency standards and how to hit 110 litres

The 110 litres water efficiency figure is an optional requirement in the Building Regulations, not a planning standard in its own right. Part G sets a mandatory limit of 125 litres per person per day for new dwellings in England. Where a local plan policy has adopted the tighter optional requirement, and a planning condition imposes it, the limit drops to 110 litres per person per day and the design has to be calculated rather than asserted. Both figures include a fixed allowance of five litres per person per day for outdoor use, so the fittings and appliances have to deliver 105 litres between them.

Where the optional standard comes from

Requirement G2 of Schedule 1 to the Building Regulations 2010 covers water efficiency. The mandatory standard is 125 litres per person per day. Regulation 36 contains the optional requirement of 110 litres, and it applies only where a planning condition says it does.

National guidance on optional technical standards sets out when an authority may impose it. The authority has to establish a clear need, usually by reference to water stress in the area, consider the effect on viability, and set the policy out in the local plan with a transition period. It is not something an officer can add to a decision notice on a whim, and a condition imposing 110 litres without a supporting plan policy is open to challenge.

Areas of serious water stress are where this bites hardest, and the number of authorities applying the tighter figure has grown steadily.

The calculator and the fittings route

Approved Document G gives two ways of showing compliance, and only one of them works for 110 litres.

The fittings approach is a table of maximum fitting specifications. Specify within it and the dwelling is treated as meeting 125 litres without further arithmetic. It is quick, and for most volume housing it is enough.

The water efficiency calculation methodology in the Approved Document is the other route. It takes the actual specification of each fitting and appliance, applies published use factors, adds the outdoor allowance and produces a litres per person per day figure. For the optional 110 litre requirement this calculation is the route, because the fittings table does not go that far.

Run the calculation at design stage rather than at handover. A sanitaryware schedule chosen on appearance and then tested against the calculator usually fails on the shower.

A specification that reaches 110 litres

The shower does most of the damage and most of the saving. Flow rate times duration times frequency dominates the total, which is why a rain head at a generous flow rate can put a dwelling over the limit on its own.

An illustrative specification that typically lands near the optional requirement looks like this, and every line of it has to be confirmed against the calculator for the particular dwelling:

1. Dual flush WC with an effective flush volume at the low end of what is available.
2. Shower at around eight litres per minute, with the flow rate verified rather than taken from a brochure for a different pressure.
3. Basin taps at four to five litres per minute, achieved with aerators or flow regulators.
4. Bath with a modest nominal capacity to overflow, since the calculator uses capacity rather than how full anyone fills it.
5. Kitchen tap at six litres per minute, bearing in mind that filling pans at a throttled flow annoys people.
6. Washing machine and dishwasher specified against the published default consumption figures in the Approved Document where the actual appliance is unknown.

Two points of friction are worth naming. Pressure matters, so a regulator sized for mains pressure behaves differently on a gravity system and the specification has to match the water supply arrangement. And the bath is often the item a client refuses to change, in which case the saving has to come from everywhere else.

Rainwater and greywater in the calculation

Rainwater harvesting and greywater recycling can be taken into account, with the deduction set by the methodology rather than chosen by the designer.

The practical difficulty is that both systems need maintenance, and a planning condition cannot sensibly require a householder to service a greywater plant for thirty years. Authorities know this, which is why fittings-led specifications are generally preferred for houses and reuse systems appear more often in blocks of flats with a managed maintenance regime.

Where a reuse system is proposed, show the storage volume, the yield calculation, the mains back-up arrangement and the maintenance responsibility. A system with no maintenance plan is a system that stops working in year three.

Water neutrality and areas of water stress

A handful of locations go further than the optional requirement. Where a water neutrality position applies, new development has to offset its demand so that total abstraction does not increase, which means the 110 litre figure is a starting point rather than the answer, and offsetting measures in existing buildings come into play.

Those positions arise from advice issued by Natural England or from local water resource constraints, and they change. Check the current position for the catchment rather than relying on a report prepared for another site.

Separately, where water supply capacity is the issue rather than resource, the undertaker's response to a pre-development enquiry is the document that matters, and that work sits in a water supply assessment rather than in the efficiency calculation.

Evidence at planning and at building control

The two regimes want different things, and applicants frequently supply the wrong one.

At planning, the authority wants a commitment and a route to it. A short section in the sustainability statement stating the target, naming the calculation methodology and giving the indicative specification is usually enough. Do not submit a full calculation for a scheme whose sanitaryware has not been chosen, because it will be wrong by the time anyone builds it.

At building control, Part G requires the calculated consumption to be notified to the building control body, with the notice given after completion within the period set out in the regulations. That calculation is based on what was actually installed.

Between the two sits the risk. A planning condition requiring 110 litres is discharged on evidence, and if the as-built specification does not reach it, the fix is replacing fittings in finished bathrooms. Where a project involves submetering across several units, the arrangements interact with the discussion in metering and submetering strategies for mixed use.

Conditions and compliance checks

Read the condition wording carefully. A condition requiring the development to be constructed to achieve 110 litres per person per day, with evidence submitted before occupation, is normal and dischargeable. A condition requiring a figure lower than the optional requirement is not supported by the regulations and should be challenged at the draft condition stage rather than after the decision.

Keep the paper trail: the design stage calculation, the sanitaryware schedule with flow rates at the design pressure, commissioning records showing measured flow rates, and the completed calculation. That bundle discharges the condition and satisfies building control with one set of work.

Draft the water efficiency section of your sustainability case on PlanScope and send it for professional review before you submit.

This is general guidance, not 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.