Air quality neutral benchmarks in London

Being air quality neutral in London is a calculation, not a statement of intent. Policy SI 1 of the London Plan 2021 requires development proposals to be at least air quality neutral, and the Greater London Authority's air quality neutral guidance sets the method: you work out the predicted emissions of nitrogen oxides and fine particulate matter from your building and from the transport it generates, then compare each against a published benchmark for that land use in that part of London. If either figure is above its benchmark, the scheme is not neutral and you either redesign or offset.

The borough case officer will look for the two comparisons set out explicitly, with the input assumptions visible. Most of the queries on these assessments are about inputs rather than about conclusions.

What air quality neutral means in policy terms

Policy SI 1 does three things at once. It requires development not to lead to further deterioration of existing poor air quality, not to create an unacceptable risk of high levels of exposure, and to be at least air quality neutral. Those are separate tests, and passing the neutral calculation does not answer the exposure question for a nursery or a care home next to a busy road.

The neutral test is deliberately relative. It does not ask whether your scheme emits nothing. It asks whether it emits more than a benchmark building of the same use and scale in the same location would. That is why a well insulated all-electric block on a main road can be air quality neutral while sitting inside an Air Quality Focus Area that still fails the national objectives.

Boroughs apply the policy through their own local plans, and several have added their own requirements on top. Check the borough's development management policies alongside the London Plan rather than treating the London Plan as the whole picture.

The two benchmarks and what sits inside each

The method splits emissions into two streams.

Building emissions cover combustion on the site: gas boilers, combined heat and power plant, standby generators and any other fuel-burning equipment. The benchmark is expressed as a mass of pollutant per square metre of floor area per year, with separate figures for nitrogen oxides and particulate matter, and it varies by land use.

Transport emissions cover the vehicle movements the development generates, derived from trip rates and the vehicle fleet assumed for the area. The benchmark again varies by use and by location, and the locational split matters more here than on the building side, because a scheme in the Central Activities Zone is compared against a baseline with far lower car use than an outer London scheme.

Take the current benchmark figures from the GLA's published guidance at the time you prepare the assessment rather than from an earlier project, because the benchmarks and the assumed fleet have been revised and will be revised again. Quoting a superseded figure is the fastest way to have an assessment sent back.

How the assessment is carried out

The sequence is mechanical once the inputs are agreed.

1. Establish the floor area by use class, measured consistently with the energy strategy so the two documents do not disagree.
2. Identify every combustion source proposed, with its rated output, its annual hours of operation and its certified emission rate.
3. Calculate annual building emissions of nitrogen oxides and particulate matter, then divide by floor area and compare with the building emissions benchmark.
4. Derive vehicle trips from an agreed trip rate source, apply the emission factors for the assumed fleet and year, and compare the result with the transport emissions benchmark.
5. Report both comparisons separately. A scheme that passes on buildings and fails on transport is not neutral.
6. Where either stream exceeds its benchmark, set out the mitigation applied and then the residual exceedance to be offset.

The most contested input is the trip rate. If the transport evidence uses one set of trip rates and the air quality assessment uses another, the borough will notice, and the discrepancy undermines both documents. Agree the trip rates once, at the start, and use them in the transport work, the air quality work and the travel plan.

Combustion plant and the emissions limits

The simplest route to passing the building benchmark is to have almost nothing to burn. An all-electric scheme with heat pumps has no building emissions of nitrogen oxides at all, which is why the policy and the GLA energy guidance now pull in the same direction.

Where combustion plant remains, the GLA sets emission standards that plant has to meet, and the limits are tighter than the manufacturer's default on much of the market. Gas boilers are the common case and ultra-low nitrogen oxide units are the expected specification. Combined heat and power is harder, because the emission rate per unit of energy is higher and the plant runs for long hours, so a scheme that gains carbon credit from combined heat and power can lose its air quality case. Standby generators are usually exempt from the annual calculation given their limited running hours, but they still need their emission rate and testing regime stated.

Quote the certified emission rate for the specific unit, with the test standard, rather than a generic figure. And keep the specification consistent with what the Energy Statement assumes, because an energy strategy built on gas and an air quality assessment built on heat pumps is an obvious contradiction.

Mitigation, and when offsetting is the answer

Mitigation on the transport side is where most schemes find the margin. Reduced car parking, car club spaces, cycle parking above the minimum standard, a delivery and servicing strategy that consolidates trips, and electric vehicle charging all reduce the modelled emissions, and all of them have to be deliverable rather than aspirational to count.

Where a residual exceedance remains, the guidance allows it to be offset through a financial contribution secured in a section 106 agreement, calculated from the shortfall. Treat offsetting as the last step rather than the plan. A borough is entitled to ask why the design could not reach neutral on its own, and an assessment that reaches for offsetting without showing the mitigation considered first invites that question.

Construction is controlled separately. Non-road mobile machinery used on sites in Greater London has to meet the Mayor's emission standards for the relevant zone, and the construction phase assessment of dust follows the industry guidance rather than the neutral benchmarks. Both belong in the submission, but neither feeds the neutral calculation.

Air quality positive for larger schemes

Masterplans and large-scale proposals subject to Environmental Impact Assessment are expected to go further, with an air quality positive approach. That is a design process rather than a calculation. It asks how the layout, massing, orientation of habitable rooms, ventilation strategy and planting reduce exposure for the people who will live and work there, and how the scheme improves conditions beyond its boundary.

In practice an air quality positive statement is produced early, feeds the masterplan rather than reporting on it, and is revisited as the design develops. Leaving it until the application is drafted defeats the purpose, because the decisions it is meant to influence have already been made. Where a site sits in an opportunity area, the expectation is firmly in this direction, and the design-led capacity work that supports optimising site capacity should carry the air quality thinking with it.

Presenting the assessment to the borough

A borough officer reading an air quality submission is checking four things: that the benchmarks used are current, that the floor areas and trip rates match the rest of the application, that every combustion source is listed with a certified emission rate, and that mitigation is secured rather than described.

Set the document out so those four can be verified without recalculation. A short table showing benchmark, predicted emission and the resulting pass or fail for each pollutant and each stream does more than several pages of narrative. State the version of the guidance used and the date of the benchmark figures. List the assumptions that would change the answer if they changed, which on most schemes is the trip rate, the fleet year and the boiler specification.

The neutral assessment normally sits inside a wider Air Quality Assessment that also covers exposure of future occupiers and the construction phase, and the two parts should not be allowed to drift apart. Where the exposure assessment recommends mechanical ventilation with filtration for frontages on a busy road, that recommendation needs to appear in the ventilation strategy and on the drawings rather than in an appendix alone.

One honest uncertainty. The transport benchmark depends on an assumed vehicle fleet that is changing quickly as electrification proceeds, so an assessment prepared against one fleet year and determined eighteen months later can look conservative or optimistic depending on which way the assumption fell. Say which year you used and why.

This is general guidance about the London planning system, not legal advice. Air quality assessment is a technical discipline and councils usually expect a suitably qualified author, so professional review is strongly advised before submission.

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