Water Supply Assessment: content and consultee expectations

A Water Supply Assessment for planning answers three questions in order. How much water will the development need, can the network deliver it, and what is being done to reduce the demand in the first place. Councils ask for one on major schemes, on sites with no existing connection, and everywhere in a water neutrality catchment, and the version that gets through validation without a query is the one that attaches the undertaker's own written response.

The mistake that costs time is treating supply and wastewater as one topic. They are separate networks, separate calculations and often separate teams inside the same company.

Baseline demand and the existing supply

Start with what the site has now. Whether there is an existing connection, where it enters the site, its size, and what it currently serves. A redevelopment replacing an office with flats may already hold a substantial connection, and existing demand can legitimately be netted off the new figure.

Then calculate the proposed demand. For housing, that is the number of units by size, an occupancy assumption, and a per person daily consumption figure. Say which consumption figure you have used and why, because the answer differs depending on whether the scheme is designed to the standard requirement or to a tighter local one.

Non-residential demand needs a different basis entirely: fittings, staff numbers, process water, cooling, irrigation and any wash-down. A mixed use scheme should present the two separately and then total them, rather than applying a residential rate to the whole floorspace.

Add a peak demand figure as well as an average. Mains capacity questions turn on peak flow, and an assessment that reports only annual volumes leaves the undertaker unable to answer.

Capacity enquiries and what the undertaker provides

Water undertakers are not statutory consultees on most planning applications, which surprises developers who expect the council to obtain the information for them. You have to ask.

A pre-development enquiry, submitted with the site location, the proposed unit numbers or floorspace and the expected demand, will normally produce a written response covering the availability of supply, the likely point of connection, whether network reinforcement is needed, and an indication of timescales. Some companies charge for it. All of them take weeks rather than days, so the enquiry goes in when the layout firms up, not when the application is being assembled.

Read the response carefully for conditional language. A reply saying supply is available subject to reinforcement is not the same as one saying supply is available, and a case officer will spot the difference.

The company's Water Resources Management Plan is useful supporting material at a strategic level. It sets out the supply and demand balance for the resource zone over a long horizon and the investment planned to close any deficit, which helps where an objection asserts there is no water available in the region.

Water efficiency standards and how they are evidenced

Approved Document G sets the water efficiency requirement for new dwellings, with a standard calculated consumption limit and a tighter optional requirement that a local planning authority can apply through a local plan policy where it has evidence of need. Where that optional requirement applies, it is imposed by planning condition and checked at building control.

Evidence it properly. The calculation uses the fittings schedule, so the assessment should list the fittings and their flow rates: taps, shower, bath capacity, WC flush volumes, dishwasher and washing machine, and any rainwater or greywater contribution allowed by the methodology.

Two practical cautions. Fittings get value-engineered between planning and completion, and a scheme approved on a 6 litre per minute shower does not comply if an 11 litre shower is installed. And rainwater harvesting or greywater reuse brings maintenance obligations and backflow protection requirements under the water fittings regulations, so a system proposed to close a calculation gap needs a management arrangement behind it.

For non-domestic buildings, water efficiency usually arrives through a sustainability or BREEAM route rather than Part G, and the credit evidence is different.

Water stressed areas and tighter local standards

The Environment Agency classifies water company areas by the degree of water stress, and authorities in the most stressed areas commonly apply the tighter optional standard as policy. Much of the south and east of England falls into that group.

Check the adopted local plan policy and the supporting text rather than assuming. Some authorities apply the tighter figure to all new homes, some only to major development, and some seek a lower figure still through a local policy that goes beyond the optional requirement, which is contestable.

Water neutrality catchments

Water neutrality is a much heavier requirement than efficiency, and it applies in a small number of places. The best known is the Sussex North water resource zone, where development has been expected to demonstrate that it will not increase overall water use in the zone.

Neutrality is achieved by offsetting: reducing demand elsewhere, usually by retrofitting water efficient fittings in existing buildings, by enough to cover the new demand. That means a costed, secured and monitored offsetting scheme, normally tied to a planning obligation, and it is a specialist exercise rather than a paragraph in a report.

Do not confuse it with nutrient neutrality, which is about phosphorus and nitrogen reaching protected habitats through wastewater, and which is a different legal route with different evidence. The distinction is set out in the comparison of nutrient neutrality and water neutrality, and the applicant's evidence burden in affected areas is covered in the note on what applicants have to show on water neutrality.

Infrastructure timing and phasing

Where reinforcement is needed, the assessment has to deal with sequence. Which works are required, who carries them out, how long they take, and what triggers them.

The Water Industry Act 1991 provides for requisitioning a main where one is needed to serve new premises, and for the duty to connect a domestic supply. Those routes exist, they take time, and they cost money. Set out the programme and propose the condition wording rather than leaving a case officer to draft something restrictive.

A Grampian style condition preventing occupation until the supply works are complete is normal and is usually acceptable to a lender, provided the works are within the applicant's control or the undertaker's published programme.

Presenting the conclusion for validation

Keep the conclusion short and make it answer the validation question directly. State the calculated demand, the standard applied, the undertaker's position on capacity, the works required if any, and the mechanism securing them.

Put the undertaker's response in an appendix in full, dated. Put the fittings schedule in a table. And where the site also raises wastewater questions, cross-refer to the Foul Sewage Assessment rather than blending the two, because the consultee, the calculation and the constraint are all different.

A Water Supply Assessment drafted to this structure gives you the policy framing and the section content. The figures still have to come from the undertaker and from the design, and the draft should be reviewed before submission.

To see which water and drainage documents a site is likely to need, enter your postcode on PlanScope and answer the guided questions.

This is general guidance, not legal advice, so 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.