Building new homes beside a busy road raises a specific planning question: will future residents be exposed to unacceptable levels of traffic pollution, and can the scheme mitigate it? An air quality assessment answers that question with data, and on roadside sites it is often the difference between approval and refusal.
This guide explains how exposure is assessed for new homes near main roads, which pollutants matter, what mitigation planners expect, and how the assessment fits current UK air quality policy.
Why roadside sites need an air quality assessment
Road traffic is the dominant source of nitrogen dioxide (NO₂) and a significant source of fine particulate matter (PM2.5 and PM10) in most UK towns and cities. Concentrations are highest at the kerbside and fall away sharply with distance, so a flat or bedroom a few metres from a main road can experience very different air quality from one at the back of the same building.
Planning has two concerns. The first is whether the development itself worsens air quality (an emissions or "air quality neutral" question). The second — the one that dominates roadside residential schemes — is exposure: are you introducing new people to a location where pollutant concentrations approach or exceed the health-based objectives? Both are covered by an air quality assessment.
The pollutants and the limits
The assessment tests concentrations against the national air quality objectives: an annual mean of 40 µg/m³ for NO₂ and PM10, and 20 µg/m³ for PM2.5, with the Environment Act 2021 targets driving PM2.5 lower still — a 10 µg/m³ annual mean target for 2040. The December 2025 revised Environmental Improvement Plan reaffirmed that direction of travel, so assessors increasingly look beyond the legacy 40 µg/m³ threshold toward the tighter PM2.5 trajectory.
For the detail on each pollutant, see our guide to NO₂, PM10, PM2.5 and NOx.
How exposure is assessed near a road
The assessor follows the EPUK/IAQM land-use planning guidance. In outline:
- Background concentrations are taken from Defra's national background maps for the site's grid square.
- Roadside contribution is added using monitoring data and, where the road is busy enough, dispersion modelling based on DfT traffic counts, fleet composition and street geometry.
- Receptor points are placed at the proposed façades and, critically, at the height and position of the nearest habitable rooms — because exposure is judged where people actually live.
- Results are compared with the objectives, and the scheme is graded from "negligible" to "substantial" in air quality terms.
If the site sits within an Air Quality Management Area, the bar is higher and mitigation is expected as a matter of course.
Mitigation that satisfies planners
Where predicted exposure is a concern, the assessment sets out mitigation. Common measures include:
- Layout and orientation — placing habitable rooms, balconies and gardens away from the roadside façade, and orienting single-aspect flats toward the quieter elevation.
- Ventilation strategy — mechanical ventilation with filtration and high-level air intakes away from the kerb, so residents are not forced to open windows onto the pollution source.
- Set-back and screening — distance, boundary planting and green infrastructure to reduce roadside concentrations.
- Emissions measures — electric-vehicle charging and low-emission travel provision to keep the scheme air-quality neutral, in line with London Plan Policy SI 1 where it applies.
When is a full assessment required near a road?
Not every roadside home triggers a detailed dispersion model. The EPUK/IAQM approach is proportionate: a small householder extension well set back from traffic may need only a short air quality statement, while a block of flats fronting a congested arterial road, a site inside an AQMA, or a scheme adding significant traffic will need a full assessment with modelling. Screening criteria — the number of new dwellings, proximity to a busy road, and whether the site lies in or near an AQMA — decide which route applies. Getting the screening judgement right at the outset avoids both an over-engineered report and a request for further information from the case officer.
Getting the assessment right first time
The most common failing on roadside sites is placing receptors in the wrong location or ignoring upper-floor exposure. Assess the worst-affected rooms, use current Defra background and DfT traffic data, and present the mitigation as an integral part of the design rather than a bolt-on. A clear, data-led assessment gives the case officer the evidence to grant consent even on a challenging frontage.
How Fortress Associates can help
Our air quality assessment service produces a site-specific report for UK planning applications using live official datasets — Defra background maps, AQMA boundaries, monitoring data and DfT traffic counts — with the report ready in minutes and free of charge. It assesses NO₂, PM2.5, PM10 and NOx against the objectives, screens AQMA and smoke-control status, and follows EPUK/IAQM criteria and Defra LAQM.TG(22). See our planning services or contact us to discuss a roadside site.
Sources & further reading
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