Water consumption targets for London residential schemes

London Plan Policy SI 5 asks residential development to achieve mains water consumption of 105 litres or less per head per day, excluding an allowance of up to five litres per head per day for external use. That figure is not an arbitrary target. It is the London Plan's way of invoking the tighter optional requirement in Part G of the Building Regulations, which is set at 110 litres per person per day including a fixed five litre allowance for outdoor use.

Understanding that relationship is the practical point of this article, because the optional requirement does not apply to a development unless a planning condition says it does. The policy, the condition and the Building Regulations calculation are three links in one chain, and schemes come unstuck when one of them is missing.

The consumption target the policy sets

For residential development the target is expressed per head per day, calculated for the dwelling rather than measured in use. A one bedroom flat and a four bedroom house are both assessed against the same per person figure, with the occupancy assumed by the calculation method rather than by the applicant.

Two things follow. The target is a design standard, so it is demonstrated by specification rather than by metering. And it is tighter than the standard that applies by default, which means a scheme specified to the ordinary Part G requirement will not comply with the London policy.

For non-residential development, SI 5 looks to the BREEAM water category and expects development to achieve the standard associated with an excellent rating in that category, or an equivalent demonstration. That is a different exercise from the residential calculation and usually sits within a wider BREEAM Pre-Assessment.

How the figure relates to the Building Regulations

Part G of the Building Regulations contains a general requirement for water efficiency in new dwellings, set at a higher consumption figure, and an optional requirement at the tighter level. The word optional is doing specific work. The tighter standard has effect only where a condition on the planning permission requires it, under the mechanism in the Building Regulations for optional requirements.

So the three figures describe the same specification from different directions. The policy states the internal figure, the Building Regulations state the total, and the condition is the instrument that converts the policy into a legal requirement on the building work.

Where a borough grants permission without the condition, the dwellings are built to the default standard and the policy is not delivered. That has happened often enough that applicants are sometimes asked to propose the condition themselves, which is worth doing, because a condition drafted by the applicant's team is likely to use the right wording and the right reference.

Fittings approach against the calculator

Approved Document G sets out two routes to demonstrating water efficiency. The fittings approach assigns maximum flow rates and volumes to each type of fitting, and a dwelling complies if every fitting meets its figure. The water efficiency calculator works out the predicted consumption from the actual specification, with weightings for each use.

For the tighter optional requirement, the calculator is the practical route. The fittings approach leaves no scope for trading a generous bath against an efficient shower, and at the tighter level that flexibility is usually needed. The calculator also allows credit for rainwater and greywater reuse, which the fittings approach does not.

What the calculator rewards:

• Low flow rate taps and showers, specified in litres per minute at a stated pressure
• Reduced bath capacity, measured to the overflow
• Dual flush or low volume WCs, with the effective flush volume stated
• Efficient white goods where they are provided as part of the dwelling
• Credited reuse systems, where the supply and demand are shown to match

The specification then has to survive procurement. A shower head substituted during fit-out, or a bath changed to a larger model, moves the calculation. Put the flow rates and volumes in the specification and in the schedule of finishes, rather than in the water calculation appendix alone.

Rainwater and greywater reuse

Reuse systems earn credit in the calculation, and they bring obligations that the planning submission should acknowledge rather than gloss over.

Rainwater harvesting is designed to BS 8515 and depends on the relationship between the collection area, the storage volume and the demand. On a tall building with a small roof and many dwellings, the yield per dwelling is small, and a system sized for the credit rather than for the demand ends up oversized and underused. On a low rise scheme with a large roof and garden irrigation demand, the arithmetic works much better.

Greywater reuse follows BS 8525 and treats water from baths, showers and basins for non-potable use. It is more effective per cubic metre because the supply is continuous, and it is harder to maintain because the treatment has to work reliably in an occupied building. Both systems need a maintenance regime, a responsible party and a water quality management plan, and both need space for plant and tanks that competes with everything else in the basement.

A reuse system also interacts with the surface water strategy. Where a rainwater tank provides attenuation as well as supply, the two functions pull in opposite directions, because attenuation wants an empty tank before a storm and supply wants a full one. Say in the Drainage Strategy which function governs, and size accordingly.

Securing the standard by condition

A workable condition does three things: it states the consumption figure, it identifies the calculation method, and it requires evidence at a stated point. Wording that asks for a scheme to be submitted and approved before occupation, demonstrating compliance with the stated figure calculated in accordance with the method in the Building Regulations, is enforceable and clear.

Avoid two patterns. A condition that simply requires the development to comply with the policy leaves the figure and the method to be argued about later. And a condition requiring a particular reuse technology locks in a solution that may not be the best answer by the time the scheme is built.

Where the scheme is phased, the condition should apply phase by phase, because a calculation prepared for the first block will not describe the specification of the third.

Evidencing compliance and the capacity question

Compliance with the optional requirement is evidenced to the building control body, not to the planning authority, and the Building Regulations require a notice of the calculated consumption to be given on completion. Keep the planning condition's evidence requirement aligned with that, so one calculation satisfies both.

Water consumption is a separate question from water capacity, and the two are regularly confused in planning submissions. Consumption is about the specification of the dwellings. Capacity is about whether the undertaker can supply the development and accept its foul flows, which is established through a pre-development enquiry to the water company rather than through a calculation. A Water Supply Assessment deals with the supply side and a Foul Sewage Assessment with the discharge side, and on a large scheme both should be commissioned early because reinforcement works can take longer than the planning process.

London sits within an area of serious water stress, which is the policy reason behind the target, and the honest position is that a 105 litre specification saves water only if the fittings installed match the ones calculated. Summarise the consumption figure, the method and the fittings specification in the Sustainability Statement, and keep that summary consistent with the energy and drainage chapters.

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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