Presenting net trip generation for a change of use

A net trip generation table change of use submissions live or die on is half a page long. It shows what the building lawfully generates now, what the proposal would generate, and the difference, for the periods the highway authority cares about. Highway officers read that table first and the supporting text second, so the table has to be self-explanatory: unit of measurement, source, periods, arrivals and departures, and a net row that anyone can check with a calculator.

What the table has to show

Six things, and nothing else in the same table.

• The land use category used for each of the existing and proposed uses, named as the trip rate database names it.
• The unit of measurement and the quantity, for example gross floor area in units of 100 m2.
• The trip rate for arrivals and departures in each period.
• The resulting vehicle trips, rounded sensibly.
• The net figure, proposed minus existing, with the sign stated.
• A footnote giving the source, the survey selection and the date of the extraction.

Keep person trips, mode share and servicing in separate tables. Mixing them in one grid is the most common reason an officer asks for the whole thing to be resubmitted.

Establishing the existing lawful use

The baseline is the lawful use, not the current occupancy. That distinction decides the outcome on most conversions, so evidence it properly: the last planning permission and its conditions, the rating list description, photographs, a statement of the use history, and floor areas measured from a survey rather than taken from a sales particular.

Where the lawful use is a class rather than a specific operation, choose the trip rate category that matches the lawful use honestly. A unit with Class E(g)(i) office consent is an office, even if the last occupier used part of it as a store, and claiming a warehouse baseline because that is what stands there invites the officer to substitute an office baseline with a much higher peak-hour rate.

A Planning Statement should set out the lawful use conclusion with its evidence so the transport note can rely on it rather than repeat it.

Choosing the periods to present

Present the periods the authority uses for its network assessment, which are usually the weekday morning and evening peak hours, plus any period specific to the use. A retail or leisure proposal needs a Saturday peak. A nursery or school run needs the actual arrival and departure windows, which do not match the highway peak. A place of worship needs its own peak, which may be Friday lunchtime or Sunday morning.

Daily totals are worth including as context, and they are the least useful line in the table, because a proposal that reduces daily trips while increasing the evening peak is still a peak-hour problem. Say which period governs and why.

An illustrative table for a conversion

The figures below are illustrative, and they show the format rather than any real trip rates. Take rates from the survey database at the time of writing with the selection recorded.

Two features make this readable. The quantity is identical on both rows, because it is the same building, which removes an argument about floor area. And the net row carries signs, so nobody has to work out which direction the change runs.

Where the proposal changes the quantity as well as the use, add a row showing the existing and proposed floor areas separately and reference the drawing they come from.

Explaining the survey source and filters

This is the part reviewers test. Record the database and version, the land use category, the geographic and population filters applied, the date range of the surveys included, the number of sites in the selection, and anything excluded with the reason.

The defensible approach is to keep the selection wide enough to be representative and to say what you excluded. Filtering to three surveys of town centre sites in one region, when forty were available, reads as selection to suit the answer even when it is justified. Where you have applied a filter that reduces the sample sharply, explain the planning logic for it, and run the numbers on the wider selection as a sensitivity.

Where person trips matter, because the site is in a location with low car ownership, use a published census-based mode share for the area and show the conversion from person trips to vehicle trips as a separate calculation.

Handling a vacant building

Long-term vacancy is the most contested baseline in this exercise. The lawful use still exists in planning terms, and most highway officers accept a lawful use baseline, but some argue for a discount where the building has been empty for years and the network has adjusted.

Deal with it directly rather than hoping nobody notices. State the date the building was last occupied, the evidence for that, and present the table with the lawful use baseline as the primary case. Then add a sensitivity showing the net position against a nil baseline, and explain why the proposal is acceptable on either reading. An assessment that survives both tests removes the argument entirely.

The national policy test is whether the residual cumulative impacts on the road network would be severe, so an honest nil-baseline sensitivity that still passes is worth more than a contested baseline that only passes on your figures.

Which document the table belongs in

On a single unit conversion, a Transport Statement containing the table, the access assessment, parking and servicing is proportionate. Larger proposals, or those crossing the thresholds the local authority applies, need a Transport Assessment with junction modelling behind the trip figures.

Where parking is the contested issue rather than trip numbers, a Parking Survey covering the streets within walking distance, carried out to the methodology the authority specifies, does more work than any trip rate table.

Common objections and how to pre-empt them

Five recur, and each has a one-paragraph answer if you write it before the objection arrives.

1. The land use category does not match the proposal. Name the category, explain why it fits, and note the alternative you considered.
2. The selection is too narrow. Report the wider selection as a sensitivity.
3. The peak periods are wrong for this use. Present the use-specific peak alongside the network peak.
4. No allowance for pass-by or diverted trips. State whether you have claimed any, and if so, the basis.
5. Servicing is ignored. Give delivery numbers, vehicle types and times in a separate table, with a swept path drawing where the yard is tight.

Write the table so that a case officer with no transport background can follow it, and the transport officer's response usually becomes a short paragraph agreeing the net position. That is the outcome worth designing for.

This is general guidance, not legal advice; trip generation evidence should be prepared by a suitably qualified transport planner and the scope agreed with the local highway authority. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.