Air quality neutral and the emissions benchmarks

Air quality neutral is a source test, not an exposure test. It asks whether the development itself emits more nitrogen oxides and particulate matter than a benchmark for that land use, and it is satisfied or failed by the plant you install and the vehicle movements you generate. That is a different question from whether future occupants will breathe acceptable air, which is what a conventional exposure based assessment answers. Schemes in London routinely need both, and submitting one while the policy asks for the other is a validation problem before it is a technical one.

The method has two halves. Building emissions from combustion plant on the site, and transport emissions from the trips the development attracts, each compared against its own benchmark.

What the test actually asks

Calculate the scheme's annual emissions of nitrogen oxides and particulate matter from the two sources. Compare each against the benchmark for the land use, floor area and location. If both sit at or below benchmark, the scheme is air quality neutral. If either exceeds, mitigate until it does not, and where that is not achievable, expect the authority to look for offsetting.

The elegance of the approach is that it does not depend on background concentrations. A scheme in a clean suburb and a scheme in the middle of an air quality management area face the same question about their own emissions, which removes the argument that a site is already polluted so a little more makes no difference.

The corollary is that passing the neutral test tells you nothing about exposure. A development can be air quality neutral and still put bedroom windows and a nursery play area beside a busy road, which is why an Air Quality Assessment looking at concentrations at receptors is usually needed alongside it. Councils normally expect that work from a suitably qualified author following recognised professional guidance, and professional review is strongly advised.

Where the requirement applies

The air quality neutral requirement comes from the London Plan and the Greater London Authority's supporting guidance, which sets the benchmarks and the calculation method. For a scheme in London, take the current version of that guidance rather than an earlier one, because the benchmarks and the land use categories have been revised since the approach was introduced.

Outside London, some authorities have adopted similar policies in their own local plans or air quality supplementary planning documents, and others rely on the national policy position and local air quality management instead. Check the adopted plan and the authority's validation list before assuming which test applies. Where the site sits in an air quality management area declared under local air quality management duties, expect the authority to ask for more, not less.

Building emissions benchmarks

Building emissions come from combustion. Gas boilers, combined heat and power, biomass and, in a smaller way, standby generators.

The arithmetic follows the plant specification, so the energy strategy decides the outcome. An all electric scheme has effectively no building emissions of nitrogen oxides or particulate matter beyond standby plant, which is why an all electric approach has become the shortest route through this policy as well as through the carbon policies. Declare any standby generator with its rated output, expected testing hours and emission characteristics rather than leaving it out, because an omission found later reads badly.

Where combustion plant is retained, the numbers turn on the emission limit of the specified units, expressed in milligrammes per normal cubic metre at stated reference conditions. Specify the limit in the submission, and be prepared for it to become a condition. A boiler selected at tender stage on price, with a higher emission rating than the one modelled, is a compliance failure that surfaces at building control and cannot be fixed with paperwork.

Biomass deserves a sentence of its own. It performs poorly on both pollutants, and in an urban authority it is very difficult to reconcile with an air quality neutral requirement whatever it does for carbon.

Transport emissions benchmarks

Transport emissions are calculated from the trips the development generates, the distance travelled, and emission factors for the vehicle fleet. The government's published emission factors are the normal source, and the version used should be stated.

Four inputs carry the result.

• Trip generation, which should be consistent with the transport evidence submitted for the same scheme
• Trip length assumptions, which have a large effect and are rarely justified properly
• Fleet composition and the assessment year, since fleet emissions change year on year
• Any existing use being netted off, which has to be established on the same basis as in the transport work

The inconsistency that reviewers find most often is a transport chapter using one trip generation figure and an air quality chapter using another. Put both in the same table and reference the transport document.

Bringing a scheme back to neutral

The order of measures is the same order as the cost of them.

1. Remove combustion plant from the design, which usually settles the building side outright
2. Where plant remains, specify low emission units and state the emission limit
3. Reduce car parking, which reduces trip generation directly rather than aspirationally
4. Provide electric vehicle charging at a level that shifts fleet assumptions defensibly
5. Secure a travel plan with mode share targets and monitoring
6. Consolidate servicing and deliveries, and restrict delivery hours where the site allows it
7. Where a residual exceedance remains, discuss offsetting with the authority early rather than at committee

Illustrative example: a mixed use scheme with a gas boiler retained for peak heating demand and 60 car parking spaces on a site with good public transport. Removing the boiler in favour of an all electric system with thermal storage takes the building side to effectively zero. Cutting parking to 20 spaces with a car club bay and full electric vehicle charging brings the transport side within benchmark. Neither change is an air quality measure in origin, and both cost less than offsetting.

Reporting format and the assumptions to declare

Present the calculation so that the reviewer can reproduce it. A results table for each pollutant against each benchmark, then an assumptions annexe covering the plant schedule with emission ratings and hours of operation, the trip generation source and figure, trip length assumptions with a justification, the emission factor dataset and version, the assessment year, the floor areas used and their source, and any existing use credit with the evidence for it.

State the version of the guidance and the benchmark set you used, and the date you downloaded it. Benchmarks are revised, and an assessment against a superseded set is a re-submission waiting to happen.

Source based emissions tests of this kind appear elsewhere in the planning system, and the discipline transfers. The approach to modelling and comparing against a threshold in ammonia deposition near designated sites uses the same logic of calculating a source term and testing it against a published figure.

Conditions that secure the outcome

Expect conditions covering the emission limits of any combustion plant, a prohibition on installing plant other than that approved without written agreement, provision of electric vehicle charging before occupation, implementation of the travel plan with monitoring reports, and a construction environmental management plan dealing with dust and non-road mobile machinery.

Two of those deserve attention at drafting stage. A condition fixing plant emission limits should reference the standard and the reference conditions, so the limit is measurable. And a travel plan condition without monitoring and without a mechanism for action if targets are missed does not secure anything.

Separately, the Energy Statement and the air quality work should tell the same story about plant. Where they disagree, the officer will assume the worse of the two. Note also that planning conditions are not the only ongoing control, since the statutory nuisance and environmental permitting regimes continue to apply after permission, as set out in statutory nuisance after permission is granted.

This is general guidance, not legal advice; check the current guidance, the adopted local policy and the position with your local planning authority or a qualified professional before relying on it.

Browse the PlanScope report types to see which documents an air quality case usually needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.