Demand calculations in a Water Supply Assessment

A Water Supply Assessment has to show two things: how much water the development will need, and whether the network can deliver it at the point you intend to connect. The water supply assessment demand calculation is the first of those, and it is built from a per capita consumption figure, an occupancy assumption, a non-domestic allowance and a separate figure for firefighting. Officers rarely challenge the arithmetic. The undertaker does, and the lead local flood authority reads the consumption target against the local plan policy.

What the assessment has to demonstrate

Three questions sit behind the document. Will the scheme meet the water efficiency standard the authority applies. Will the existing network deliver the peak demand at the connection point. And does the supply meet the flow requirement for firefighting.

Those are answered by different people. The first is a Building Regulations and local plan matter. The second belongs to the water undertaker. The third involves the fire and rescue service as well as the undertaker.

Keeping the three separate in the report structure stops the common failure, which is a document that proves the dwellings are efficient and says nothing about whether the main in the road is big enough.

Per capita consumption and the fittings approach

Part G of the Building Regulations sets a water efficiency requirement for new dwellings of 125 litres per person per day. There is also an optional tighter requirement of 110 litres per person per day, which a local planning authority can apply through an adopted local plan policy where there is clear local need.

Compliance is demonstrated by the water efficiency calculation method set out under Part G, using the flow rates and capacities of the fittings specified. That means the calculation is a schedule of taps, showers, baths, WCs and appliances rather than a single assumption.

The fittings approach has a practical consequence. A bath with a large capacity can push a dwelling over the target on its own, and the usual fix is a smaller bath volume or a lower-flow shower rather than anything structural. Do that specification work before the Sustainability Statement is written, because both documents quote the same figures and they have to agree.

Where the scheme sits in an area the Environment Agency classifies as seriously water stressed, expect the tighter target and expect the officer to look for it. Some authorities go further through policy and ask for a lower figure again, supported by rainwater or greywater reuse.

Occupancy and the conversion to a daily volume

Consumption is per person. Demand is per site. The bridge between them is an occupancy assumption, and it should come from a source you can name, usually the undertaker's own design guidance or the local housing needs evidence.

State the assumption, the unit mix and the multiplication. A reviewer who can see the arithmetic is not going to argue with the answer.

Peak demand then matters more than the daily average. Networks are sized for the morning peak, and the undertaker applies its own peaking factors, which are steeper for small populations than for large ones.

Non-domestic demand and irrigation

Commercial and community uses do not scale from a per capita figure. A gym, a nursery, a food unit and a laundry each have distinct profiles, and the sensible approach is to take demand from the occupier's own equipment schedule where one exists.

Irrigation is the quiet addition on landscape-led schemes. Podium planting, green roofs and new street trees all need water in establishment years, and on a large scheme that is a real volume rather than a rounding error. Say how it will be supplied, and whether it is metered separately.

Trade effluent is the mirror image on the wastewater side, and it needs its own consent from the undertaker. The wastewater half of the same enquiry is covered in the drainage hierarchy section of a drainage strategy, and the two enquiries should be sent together.

Fire flow and hydrant provision

This is the section most often missing. Water undertakers have duties in relation to fire hydrants under the Water Industry Act 1991, and the fire and rescue service has its own expectations for flows and hydrant spacing depending on the type and size of building.

The practical questions are whether the main can deliver the required flow at the required pressure, whether new hydrants are needed, and who pays for them. On a scheme with a tall residential block, sprinkler and fire main requirements under the Building Regulations add a further demand that has to be checked against the same network.

Ask the undertaker and the fire and rescue service in parallel. Their answers arrive on different timescales and the fire service response is often the slower of the two.

Network capacity and what the undertaker sends back

A pre-development enquiry to the water undertaker returns the network records, an indication of available capacity and any reinforcement required. It is the same process as the foul enquiry, and it is worth running both at once. The Foul Sewage Assessment uses the same occupancy assumption, so make sure the two documents do not quote different numbers.

Where reinforcement is needed, the usual planning outcome is a condition preventing occupation until the works are complete or an agreement is in place, exactly as with foul drainage. The Water Supply Assessment should set out the programme for that work rather than leaving it to the discharge of conditions stage.

Conditioning the specification so it survives to completion

A water efficiency figure in a report does nothing unless it reaches the plumber. Authorities usually condition the specification, and the sensible wording requires the approved fittings schedule to be installed and retained.

Three things are worth doing before submission.

1. Agree the target figure with the case officer by reference to the adopted policy, not to national minimum standards.
2. Fix the fittings schedule at the same time as the planning drawings, so the Part G calculation and the submitted specification match.
3. Get the undertaker's capacity response in writing and attach it, so the officer has no reason to ask a further question.

An illustrative case. A 60-unit scheme in a seriously water stressed area is designed to 125 litres per person per day because that is the Building Regulations figure the design team knew. The adopted local plan requires 110. The fix is a revised fittings schedule and a reissued calculation, which is a fortnight of work if it is caught at validation and a great deal more if it is caught at committee. The figures are illustrative, the sequence is not unusual.

And note the honest limitation. A demand calculation is a design estimate. Actual consumption varies with occupancy and behaviour, which is why the condition attaches to the fittings rather than to litres used.

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.