Net trip generation on a change of use application

When a highway officer asks for trip generation on a change of use, the number that matters is the net trip generation change of use figure, not the gross one. The proposal is not adding traffic to an empty site. It is replacing whatever the lawful existing use generates, and the comparison between the two is the whole of the transport case for most small commercial applications. A takeaway replacing a bank looks alarming as a gross figure and often looks negligible as a net one.

Getting the credit for the existing use is not automatic. You have to establish what the lawful use is, show that it could realistically resume, and compare like with like.

Why the existing use matters

Highway authorities assess the impact of the development on the network. If the site already has a lawful use capable of generating trips, the impact is the difference, and the baseline is the lawful position rather than the currently occupied one.

That principle has real force where the existing use is trip-intensive. A 250 m² unit in lawful retail use generates a substantial number of vehicle and pedestrian arrivals across a day. Replacing it with a use that generates fewer is, in transport terms, an improvement, and saying so with evidence reframes the whole consultation response.

It has less force where the existing use is low intensity. A change from storage to a restaurant does not get much of a credit, and pretending otherwise wastes credibility you will need later.

Establishing the lawful existing use

Start with the planning history. The last permission granted for the unit, any lawful development certificate, and the description of development on each decision notice.

Where the history is silent, the evidence that works is documentary and dull. Business rates records showing the rateable description and the period of assessment. Lease or tenancy agreements. Utility accounts. Dated photographs. Accounts or till records where the intensity of the use is also in question.

Two complications. Where a use has continued without permission for long enough it may have become immune from enforcement, and the enforcement time limits in England were changed by the Levelling-up and Regeneration Act 2023 towards a single ten year period. Check the commencement position and the dates that apply to your breach before relying on immunity.

And where the unit has stood empty for years, the question becomes whether the use has been abandoned. Abandonment is a matter of fact and degree, looking at the physical condition of the building, the length and circumstances of the non-use, any intervening use and the owner's intentions. An abandoned use generates no credit at all.

A certificate of lawfulness for an existing use is the only formal way to put the point beyond argument. On a contested site it is worth the fee.

The Class E point

Movements within Class E are not development. A unit lawfully in Class E can be a shop, a cafe, a gym, an office, a nursery, a clinic or a light industrial workshop without any application, and a highway authority has no opportunity to ask for a transport assessment at all.

So when permission is needed to move from Class E to a sui generis use such as a hot food takeaway, a drinking establishment or a betting shop, the baseline is not the use currently trading. It is any use within Class E that could lawfully be implemented tomorrow.

That is a strong argument and it is frequently missed. Pick the Class E use with the highest trip generation that is realistic for the unit, evidence why it is realistic, and run the comparison against that. Then run it against the current use as well, so the officer can see both.

Trip rates for the existing and proposed uses

The recognised source is the national trip rate database, filtered by land use category, region, location type and survey date. The filtering is where the argument lives.

State every filter you applied and how many survey sites remain. An assessment built on three surveyed sites from one region in 2009 is not evidence, and a highway officer will say so. Where the sample is thin, say it is thin and present a sensitivity rather than hiding it.

Use gross floor area consistently, and say whether it is gross internal or gross external, because trip rates are published against a defined basis. Mixing the two is the arithmetic error I see most often.

For uses with no reliable trip rate data, a first principles calculation based on staff numbers, customer throughput and delivery frequency is acceptable if the inputs are stated and justified by the operator. Say that the operator provided them.

Peak hour comparison rather than daily totals

Compare on the network peak hours and on the development's own peak, and present both.

Daily totals mislead in both directions. A hot food takeaway generates most of its trips in the evening, when the network is quieter, so a daily comparison overstates the problem. A nursery generates a short, intense peak at 08:15 that a daily figure hides entirely.

Set out arrivals and departures separately rather than combining them into two-way movements, because the junction assessment needs the turning movements. And identify the vehicle mix. Twelve car trips and twelve refuse or delivery vehicle trips are not the same impact.

A short table is worth more than three paragraphs.

Where the net difference is negative

A negative net figure is a good outcome and it does not end the assessment.

The distribution of trips may change even if the number falls. A use drawing customers from a wider catchment puts traffic on different arms of a junction. The parking demand profile may shift from short stay to long stay, which matters more on a street with limited kerb space than the raw count does. Servicing may move from a weekly delivery to daily ones, and the vehicle size may increase.

Deal with those in the narrative rather than leaving the negative number to carry the whole case. Where parking is the contested issue, an evidence-based survey is usually more persuasive than a standards comparison, which is the point of a Parking Survey.

Whether the submission ends up as a Transport Statement or a fuller Transport Assessment depends on scale and on the local validation list, and the thresholds are set out in transport assessment thresholds and trip generation.

Presenting the comparison

Put the net table on the second page, not in an appendix. Above it, one paragraph stating the lawful use relied on and the evidence for it. Below it, the caveats.

Then offer something. A servicing management plan restricting delivery times, a staff travel information pack, or a condition limiting opening hours costs little and gives the highway officer a reason to withdraw an objection they have already drafted.

Transport work is normally expected from a suitably qualified author and professional review is strongly advised before submission.

Enter your postcode on PlanScope to see which reports a change of use proposal 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.