Water neutrality evidence in a water supply assessment

Water neutrality evidence has to show that total water use in a defined supply area is no higher after your development is occupied than it was before. That is the test, and it is harsher than water efficiency. A scheme can meet every efficiency standard in the Building Regulations and still fail, because the new homes add demand that was not there before. The evidence therefore has two halves: a defensible calculation of the new demand, and a secured mechanism that removes at least as much demand from somewhere else in the same supply area.

The requirement does not come from planning policy. It comes from the Conservation of Habitats and Species Regulations 2017, through an appropriate assessment where Natural England advises that abstraction in a supply zone may be affecting a European site.

Where the requirement applies and why

The established English example is the Sussex North water supply zone, where Natural England advised in 2021 that abstraction could not be shown to have no adverse effect on the integrity of the Arun Valley designated sites. Local planning authorities in the affected area were left unable to conclude an appropriate assessment for new development that added water demand, so schemes had to demonstrate neutrality instead.

That mechanism can appear anywhere the same chain of reasoning holds. Check the position with the local planning authority and with Natural England before you assume neutrality is or is not engaged, because the advice is catchment-specific and it changes. If the site is outside an affected zone, the assessment is an ordinary water resources chapter and not a neutrality case.

The conclusion lands in the authority's Habitats Regulations assessment, not in your report. Your job is to give the authority evidence it can adopt.

Calculating the new demand

Start from occupancy rather than from unit numbers. A neutrality budget that assumes one person per bedroom will be challenged, and rightly. Use the occupancy assumptions the authority has published for its own strategy, or a recognised national dataset, and state which you used.

Then set the per person consumption figure the scheme will achieve. Part G of the Building Regulations sets a standard of 125 litres per person per day, with an optional tighter requirement of 110 litres per person per day that a council can apply by condition where a local plan policy justifies it. Neutrality schemes commonly go further, to around 85 to 100 litres per person per day, using a fittings specification rather than rainwater or greywater systems, because fittings are easier to verify and less likely to fall out of use.

Non-household demand needs its own line. Commercial floorspace, a care home, a school and a hotel all behave differently from dwellings, and an occupancy-based figure will not transfer. Where the end occupier is unknown, present a worst credible case and say so.

The budget should also include external water use, communal landscaping and any construction phase demand that draws on the same supply. Those are frequently omitted, and an officer who spots the omission will treat the whole calculation as optimistic.

Offsetting through retrofit and its evidence

Most offsetting works by retrofitting water-efficient fittings into existing buildings in the same supply area. The arithmetic looks simple and the evidence rarely is. You are claiming a saving from a property you do not own, occupied by people whose behaviour you cannot control, using a baseline you have to establish.

A defensible retrofit package sets out the property type, the number of occupants, the fittings replaced, the flow rates before and after, and the saving claimed per property. It also states the assumed persistence, meaning how long the fitting is assumed to remain in place, and applies a discount for fittings that are removed, damaged or defeated. A claimed saving that assumes permanence for thirty years with no discount will not survive scrutiny.

Where an authority runs its own offsetting scheme or credit pool, buying into it is usually faster and more certain than assembling your own retrofit programme. Ask whether credits are sold, at what rate, and whether the scheme has been through the authority's own appropriate assessment.

Rainwater harvesting and greywater recycling can contribute, but be careful how much you claim. Their yield depends on maintenance and on the behaviour of the occupier, and several authorities discount them heavily for that reason.

Fittings specification and monitoring

The specification is the part of the report that ends up in a condition, so write it as though it will be. List the fitting types, the maximum flow rate or capacity for each, and the calculation method you used to reach the per person figure. The water efficiency calculator used for Part G compliance is the usual method, and using anything else invites an argument.

Two practical points. Dual flush cisterns underperform their nominal rating where they are poorly maintained, so some assessors apply a reduction. And a specification that relies on flow restrictors at the outlet is weaker than one that relies on the fitting itself, because restrictors are the first thing a frustrated occupier removes.

Set out how compliance will be demonstrated at completion. A schedule of installed fittings, with product references and flow rates, signed off before occupation, is the usual expectation.

Securing offsetting in an obligation

Neutrality offsetting is almost always secured by a section 106 obligation rather than by condition, because it involves works on third party land and payments to a scheme. The obligation needs to identify the offsetting mechanism, the quantity of saving secured, the timing relative to occupation, and what happens if the saving is not delivered.

Timing is the point most often argued. Offsetting delivered after occupation leaves a period where the development is adding demand with nothing removing it, which an authority may treat as a failure of the neutrality test rather than a programming detail. Expect to be asked to deliver the offsetting before first occupation, or in phases tied to occupation triggers.

Where the offsetting sits inside a council-run scheme, the obligation may simply be a payment plus a covenant not to occupy until the payment is made and the credits allocated. Set out that chain clearly in the assessment so the officer can see it works. A Water Supply Assessment that stops at the calculation and leaves the securing mechanism to the legal team usually comes back.

Verification after occupation

Authorities increasingly want to know what happened after the homes were sold. Verification can take the form of a fittings audit on a sample of plots, consumption data supplied by the water undertaker where the occupiers consent, or a report from the offsetting scheme confirming the retrofits were completed.

An illustrative example. A scheme of 48 homes in an affected supply zone calculates a demand of 85 litres per person per day against an occupancy figure taken from the authority's published strategy. The offsetting is bought as credits from the council's retrofit programme, secured by a section 106 obligation with delivery before first occupation, and verified by a completion schedule of installed fittings plus confirmation from the programme that the matching retrofits are in place. That package is short. It is also complete, which is the quality the officer is looking for.

What to submit alongside it

Neutrality evidence rarely travels alone. Expect to submit it with a Drainage Strategy and, where the site is not on mains drainage or the receiving works are constrained, a Foul Sewage Assessment. Where a local plan sets wider resource efficiency targets, the same fittings specification usually appears in the Sustainability Statement as well, and the figures in the two documents must match.

Ask the water undertaker early whether it has capacity concerns of its own. A neutrality case and a capacity constraint are different problems with different solutions, and answering one does not answer the other.

This is general guidance, not legal advice. Check the position with your local planning authority, Natural England and a suitably qualified water resources specialist before relying on it.

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