Netting off an existing use in trip generation
An existing use credit only survives if the lawful use is still extant. Here is the evidence councils expect, how to match the survey periods on both sides of the subtraction, and where highway officers refuse a credit.
An existing use credit in trip generation only works if you can prove the existing use is lawful and still alive. That is the whole of it. Highway officers are not resisting the arithmetic when they refuse a credit for a change of use, they are resisting the claim that the unit is still, in planning terms, a shop or a workshop or an office with all the vehicle movements that implies. So the transport work starts with planning history, not with trip rates.
Get the lawful use nailed down, evidence that it has not been abandoned, then net off. In that order.
Establishing the lawful use before you count anything
The lawful use is what the planning permission, or the absence of one, actually allows. Three routes get you there.
A permission with a description or a condition tying the unit to a named use is the cleanest. A unit within a use class under the Town and Country Planning (Use Classes) Order 1987 as amended gives you the class, which matters because movement within Class E needs no permission at all and therefore no transport case. Where the use grew up without permission and has continued long enough to be immune from enforcement, a Lawful Development Certificate under section 191 of the Town and Country Planning Act 1990 converts an argument into a document.
Officers read conditions closely here. A permission for a builder's merchant with a condition restricting deliveries to two vehicles a day does not give you a general Class B8 trip baseline, and a restaurant permission with a condition limiting covers caps the trips you can claim.
Evidence that the use is extant rather than historic
Planning law treats a use as capable of being abandoned. Where it has been, the credit disappears along with it, and the fallback becomes whatever the site can lawfully do now. Four considerations have been applied consistently in the case law: the physical condition of the building, how long it has been out of use, whether any other use has intervened, and what the owner's intentions appear to have been.
Assemble the paper before you write a word of transport analysis.
- Business rates records showing the rated use and the liable party by year
- Utility bills or metered consumption records showing use consistent with occupation
- Audited accounts, VAT returns or payroll records for the trading entity
- Delivery notes, waste transfer notes and trade waste contracts
- Tenancy agreements, licences and any break notices
- Dated photographs, and dated aerial or street level imagery with the capture date recorded
- A statutory declaration from the operator where records are thin
Photographs of a stripped-out unit with the shutters down are the single most damaging item in a pack, so deal with them head on if they exist.
Counting the existing trips honestly
Where the use is still operating, survey it. A classified turning count and a car park occupancy count at the site itself beats any database, and no officer argues with a count they can check against their own knowledge of the road.
Where it has stopped trading but remains lawful, the TRICS database is the normal fallback. The discipline is in the filter set rather than the output. Record the land use category, the region and subregion selected, the date range, the days of the week, the survey count, whether you excluded Ireland, and how you treated multi-modal sites. Then show the rank order of the surveys you kept, not just the mean.
Two traps recur. The first is comparing a gross floor area for the existing use with a net internal area for the proposal, which quietly inflates the credit. The second is using an average weekday rate for the existing use and a peak hour rate for the proposal. Keep the unit of measurement, the time period and the day type identical on both sides of the subtraction, and say in the text that you have done so.
Illustrative example: a vacant 420 m² retail unit becoming a gym. Retail trip rates peak on a Saturday afternoon while a gym peaks at 7am and again after 6pm, so a net figure across a twelve hour day can show a reduction while the evening network peak shows a clear increase. Presenting both periods is the difference between a credible submission and a request for further information.
Vacancy and what it does to the baseline
There is no national rule that a credit expires after a set number of years of vacancy. What happens in practice is that the longer the vacancy, the harder the officer looks at abandonment, and the more likely the authority is to ask for a discounted baseline.
Three positions you will meet. Some authorities accept the full lawful use baseline regardless of vacancy, on the basis that the use could resume tomorrow without permission. Some apply a discount, often justified by reference to the proportion of similar units in the centre that are trading. Some refuse any credit where the unit has been empty for years and the interior has been stripped.
None of those positions is written down in national policy, which is why the supporting text has to do the persuading. Where a discount is likely, model it: show the net effect at full credit, at half credit and at nil credit, and demonstrate which of those outcomes would actually cause a problem on the network. If the answer is none of them, the argument about the credit stops mattering, which is the best place to be.
Presenting net figures without hiding the gross
Put the gross figures in the table. A submission that only ever reports net movements looks like a submission with something to conceal, and it invites a request to see the workings.
| Row to show | Why the officer wants it |
|---|---|
| Existing use, gross two-way trips by peak hour | Establishes the claimed baseline openly |
| Proposed use, gross two-way trips by peak hour | Allows an independent check against TRICS |
| Net change by peak hour | The figure the policy test is applied to |
| Net change with no credit applied | Shows the worst case without a second round of questions |
| Mode split assumptions | Explains any difference between person trips and vehicle trips |
A Transport Statement is usually the right container for a small change of use, with a full Transport Assessment reserved for schemes at or above the scale where the authority's own threshold bites. The thresholds and the trip generation method behind them are set out in more detail in our guide to transport assessment thresholds.
Where the highway authority will push back
Netting off does nothing for a safety objection. If the access geometry is substandard, or visibility splays cannot be achieved, or refuse vehicles cannot turn, the fact that a previous use generated more trips is beside the point. Deal with those matters on their own terms.
The other common resistance is about the shape of the demand rather than its volume. A change from office to nursery can reduce total daily trips while concentrating arrivals into a fifteen minute window outside a school gate. A change from warehouse to trade counter reduces HGV movements but multiplies car movements and on-street parking demand, which is when a Parking Survey earns its place in the pack.
The assumptions log that saves you a round of comments
Write a short annexe that a stranger could reproduce. Name the source of the existing floor area and the drawing it came from. State the lawful use and the document that establishes it. Give the TRICS filter set in full, or the survey date and weather for a site count. Record any discount applied to the credit and the reason for it. List what you did not do and why, such as omitting a servicing analysis because the proposal uses the existing loading bay unchanged.
That annexe is also what makes the file defensible two years later, when the same site comes back with a section 73 application and nobody remembers how the baseline was built.
This is general guidance, not legal advice. Whether a use is lawful and extant is a matter of fact and degree for the local planning authority, so check the position with the authority or a qualified professional before relying on a credit. Enter your postcode on PlanScope to see which reports your site is likely to need.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Can I claim trips from a use that stopped years ago?
Sometimes, but the longer the gap the harder it gets. A use can be abandoned in planning law, and the tests applied are the physical condition of the building, the length of non-use, whether another use intervened and the owner's apparent intentions. Assemble rates records, bills, accounts and dated photographs before you rely on the baseline.
What proves the existing use is lawful?
A planning permission describing the use, membership of a use class under the Use Classes Order 1987 where no permission is needed, or a Lawful Development Certificate under section 191 of the Town and Country Planning Act 1990. Read any conditions attached, because a restriction on deliveries or covers limits the trips you can claim.
Should I survey the site or use TRICS?
Survey it if the use is still trading, because a count at the site itself is harder to argue with. Where trading has stopped, TRICS is the normal fallback, and the filter set matters more than the output. Record the category, region, date range, day types, survey count and rank order in an annexe.
Why does the highway authority want gross figures too?
Because net figures alone cannot be checked. Showing existing gross, proposed gross, net change and the net change with no credit applied lets the officer verify your arithmetic and see the worst case in one pass. It also removes the most common reason for a request for further information.
Does a reduction in total trips settle a highway objection?
No. Safety objections about access geometry, visibility or refuse vehicle movements are unaffected by the volume of trips. Nor does a lower daily total answer a concern about the shape of demand, such as a nursery concentrating arrivals into a short window on a residential street.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.