Water supply evidence in a water stressed area

In a water stressed area supply evidence has two separate jobs. One is to show that water can physically be delivered to the site, which is a conversation with the water undertaker. The other is to show that the development will use less of it, which is a calculation against a tighter efficiency standard that many authorities apply through local policy. Developers routinely do the first and forget the second until a condition appears on the decision notice.

The Environment Agency publishes a classification of areas of water stress, and the water companies serving those areas face the greatest supply pressure. That classification is the trigger for most of what follows.

What a water stress designation changes

The designation itself does not prohibit anything. It changes three things in practice.

It gives the local planning authority the evidence base to apply the tighter optional water efficiency standard through local plan policy, because that standard requires a clear local need to be demonstrated.

It makes the water undertaker's consultation response more likely to raise capacity and reinforcement issues, and more likely to seek phasing conditions tied to infrastructure delivery.

And in a small number of catchments it goes further, where the advice from the nature conservation body has created a water neutrality requirement, meaning the development must not increase total water abstraction from the affected source at all.

Establish which of those three applies to your site before commissioning anything. The evidence for a straightforward site in a stressed area is modest. The evidence for a site in a water neutrality area is a project in itself.

The optional tighter water efficiency standard

Part G of the Building Regulations sets a mandatory whole building standard for new dwellings, expressed as litres per person per day, with a tighter optional requirement available at 110 litres per person per day.

The optional standard does not apply automatically. A local planning authority has to adopt a policy requiring it, supported by evidence of local need, and then impose it on individual permissions by condition. The compliance itself is then demonstrated through the Building Regulations process rather than through planning.

So the planning task is narrow and specific. Confirm whether the adopted local plan requires the optional standard. If it does, state in the submission that the development will achieve it, and set out how. If it does not, say so, because an unnecessary commitment made in a planning statement can end up conditioned.

Where the scheme is non-residential, the equivalent levers are different and usually come through a sustainability assessment or a certification scheme, with water consumption credits assessed in a BREEAM Pre-Assessment by a licensed assessor.

Fittings and fixtures that deliver it

The calculation is a fittings exercise, and the figures are decided by specification rather than by behaviour.

The Part G water efficiency calculator takes the flow rate or capacity of each fitting and combines them into a whole dwelling consumption figure:

• taps, by flow rate in litres per minute, with basin and kitchen taps assessed separately
• showers, by flow rate, which is usually the single largest item
• WCs, by the effective flush volume of a dual flush cistern
• baths, by capacity to the overflow
• washing machines and dishwashers, by litres per cycle where they are fitted as part of the dwelling

Reaching the tighter figure is normally achieved through a low flow shower, dual flush WCs and restricted basin taps, with the bath capacity as the variable that causes arguments with the sales team. There is no need for anything exotic in most cases.

Two cautions. Specify the fittings on a schedule and keep that schedule consistent with the calculation, because a substitution on site invalidates the figure. And resist describing the standard as a sustainability benefit in the planning statement if it is simply compliance with the adopted policy, since officers notice the difference.

Water neutrality where it applies

In the few catchments where water neutrality applies, the test is quantitative and the burden is heavier.

The approach is to establish the development's predicted demand, then demonstrate offsetting savings elsewhere so that total abstraction does not increase. Offsetting usually means retrofitting water efficiency measures in existing buildings, which raises the questions of who delivers them, how they are secured, how they are verified and how long they endure.

The evidence set typically includes a demand calculation for the proposed development on a stated basis, an offsetting strategy naming the measures and the properties, a delivery mechanism through a planning obligation or a local offsetting scheme, and a monitoring and verification proposal.

Treat it as a specialist exercise with its own programme. Where it applies, it is usually the longest lead item in the application, and schemes have waited years for an offsetting mechanism to become available.

Rainwater and greywater reuse evidence

Reuse systems can contribute to the calculation, and the calculator makes allowance for them, but they bring their own evidence requirements.

Three things to get right. The storage volume needs a yield calculation based on roof area and local rainfall, not a manufacturer's generic figure. The overflow has to connect somewhere, which brings it into the surface water strategy and therefore the Drainage Strategy, where the allowances applied are explained in our note on climate change allowances in a drainage strategy. And maintenance has to be described, because a system that is not maintained delivers nothing and councils have become sceptical of unmanaged installations in individual dwellings.

For apartment schemes with a managed estate, reuse is more defensible than on a row of houses where each owner is responsible for a filter they will never change.

Undertaker consultation responses

Approach the water undertaker early with a pre-development enquiry giving the number of units, the type of development, the expected occupancy and the connection point.

Four responses are common, and each has a different consequence.

Capacity available, connect on application. The simplest outcome, which still needs the connection cost in the budget.

Capacity available subject to local reinforcement, usually at a stated cost or through a requisition process under the water industry legislation. This is a budget item and a programme item.

Capacity constrained, with delivery tied to a future scheme in the company's investment programme. This can produce a phasing condition linking occupation to infrastructure delivery, which affects cash flow materially.

A request for the development to achieve the tighter efficiency standard, which the undertaker cannot require directly but will often recommend to the authority.

Record the enquiry reference and the response in the Water Supply Assessment, with the dates. A capacity position stated without a reference is the most common reason a consultee asks for the assessment to be revisited.

Conditions securing the standard

Expect one of three condition forms, and read the wording carefully.

A condition requiring the dwellings to be constructed to achieve a stated consumption figure, with compliance evidenced through the Building Regulations. Clean and testable.

A condition requiring a water efficiency statement or fittings schedule to be submitted and approved before works above slab level. More onerous, because it is a discharge application with a fee and a determination period.

A condition requiring reuse systems to be installed in accordance with approved details and thereafter maintained. The maintenance obligation runs with the land, so make sure the management arrangement exists.

Where the scheme includes a wider sustainability narrative, keep the water section short and factual within the Sustainability Statement rather than repeating the calculation in three documents. Consistency between documents is what consultees check first.

And watch for change. The standards in this area have been under review for several years as part of wider work on building performance, so verify the current requirement rather than copying a figure from a previous application.

Check the adopted local plan for the optional standard before you specify a single fitting. Enter your postcode on PlanScope to see which reports a site in a water stressed area is likely to need.

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.