Capacity calculations in a utilities assessment

Capacity calculations in a utilities assessment exist to answer one question for the planning officer and a different one for the developer. The officer wants to know that the scheme can be served without unacceptable off-site works. The developer wants to know the cost and the lead time. Both answers come from the same arithmetic: estimated demand by use, a diversity assumption, a point of connection confirmed by the undertaker, and the reinforcement that follows.

Get the estimate wrong at planning stage and the consequence is rarely a refusal. It is a substation appearing on a layout eighteen months later, in a position that needs a new application.

What the assessment is for at planning stage

A Utilities Assessment at application stage is a feasibility document, not a design. It establishes that electricity, water, foul drainage, gas where relevant and telecommunications can be provided, identifies the physical works needed, and reserves space in the layout for anything with a footprint.

That means three outputs. A demand estimate per utility with the assumptions stated. A point of connection for each, with evidence from the undertaker. And a schedule of works, constraints and lead times, with anything affecting the layout drawn.

Keep it separate from the drainage documents. Surface water belongs in the Drainage Strategy and foul capacity in the Foul Sewage Assessment, and a utilities assessment that duplicates them creates contradictions between documents.

Load estimates by use and diversity factors

Electrical demand is the calculation that matters most, because it drives the connection cost and the substation requirement. Estimate it from the schedule of accommodation rather than from floor area alone.

For housing, network operators apply an after diversity maximum demand per dwelling, published in their own design standards. Those figures have risen as heating and cooking have electrified, and a scheme with heat pumps and charge points carries a materially higher demand per home than the gas-heated equivalent. Use the operator's current figure rather than one from an older design guide, and say which you used.

For commercial and mixed use floorspace, work from a watts per square metre allowance by use, then apply diversity. State both numbers. An assessment giving a single total with no breakdown cannot be checked and will be queried.

Four assumptions should be visible in the document:

• the demand per dwelling or per square metre, with its source
• the diversity factor applied and at what level in the network
• whether electric heating, cooking and vehicle charging are assumed
• whether load management or a demand side arrangement is assumed for charging

Water demand follows a similar pattern, estimated per dwelling per day with peak flow factors, with the undertaker's own assumptions preferred. Foul flows are derived from water demand with an allowance for infiltration where the network is old.

Points of connection and who confirms them

An estimate is not evidence. The evidence is a written response from each undertaker identifying where the connection can be made and what it requires.

Each utility has its own enquiry route and its own product names, and the useful distinction is between an indicative response and a formal offer. A budget estimate or capacity statement is enough for a planning application. A formal connection offer, which is a priced contract with a validity period, is what you need before fixing the programme.

Submit the correspondence as an appendix. Officers and statutory consultees take an undertaker's own words far more seriously than a consultant's summary of them, and the correspondence also dates the enquiry, which matters when capacity changes.

Reinforcement requirements and lead times

Reinforcement is work on the wider network needed because your demand exceeds what the local network can currently take. It is the item that turns a modest connection into a long programme.

What is chargeable to the connecting customer has shifted through successive charging reforms, so read the current offer rather than assuming the position from an earlier scheme. The planning-relevant consequences are physical and temporal: a new substation needs a site, a new main needs a route and street works, and both need time.

Record lead times as a range with a source, which is usually the undertaker's own stated programme. And flag the single longest item, because that sets the critical path. On larger schemes it is often a primary substation or a sewer upgrade rather than anything on the site itself.

Where phasing is proposed, show which phase triggers which reinforcement. A scheme consented in phases with a single connection assumption tends to discover that phase three cannot be energised, which is a version of the problem described in utilities capacity evidence for a residential scheme.

Diversions and easements

Existing apparatus crossing the site is a layout constraint before it is a cost. Obtain asset plans from every undertaker at the outset, plot them on the site plan, and treat each as a corridor rather than a line, because asset plans are indicative and the actual position has to be confirmed by trial holes.

Then decide, for each one: design around it, build over it with the undertaker's agreement, or divert it. Diversion rights and procedures differ by utility, with water and sewer diversions under section 185 of the Water Industry Act 1991, electricity under the Electricity Act 1989 and the operator's procedures, and telecoms under the Electronic Communications Code.

Easements over third party land take months and need a willing neighbour. Where the only viable route to a point of connection crosses someone else's land, say so in the assessment rather than leaving it to be discovered at the contract stage.

Evidence to include from each undertaker

A document a consultee can check contains: the enquiry submitted, the response received, the dates of both, the point of connection shown on a plan, the stated capacity position, any reinforcement identified, the works required on and off site, and the lead time.

Two habits improve it. Date every statement, because capacity is a moving position and an undated capacity statement is worthless two years later. And distinguish clearly between what the undertaker has confirmed and what the consultant has assumed, using separate paragraphs rather than blended prose.

Where an undertaker has not responded in time for submission, say that plainly, state when the enquiry was made, and propose a condition. Officers accept that far more readily than a confident assertion with nothing behind it.

Where the assessment affects the layout

This is the section developers skip and then regret. Four things need space on the drawings at application stage.

A substation, with the operator's required dimensions, vehicular access for a crane or trailer, clearances and often a requirement that it sits on adoptable frontage rather than inside a courtyard. A water booster or meter housing where pressure requires it. A pumping station for foul drainage where gravity does not work, with its own buffer to dwellings. And duct routes and service corridors wide enough for all utilities with their mandatory separation distances, which on a tight site competes directly with tree root protection areas.

Put all of them on the planning layout. A substation added by condition discharge in the only remaining corner is how schemes lose a parking space, a bin store or an affordable unit.

And state what has not yet been confirmed. An assessment that acknowledges an open item with a route to resolving it is more credible than one that implies certainty the undertakers have not given.

This is general guidance, not legal advice. Design standards, demand assumptions, charging arrangements and lead times are set by each undertaker and revised periodically, so check the current published position and confirm requirements 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.