Air Quality Assessment for a 250-Home Scheme in an AQMA

The Project

A regional housebuilder was bringing forward a brownfield site for 250 dwellings in a North West town. The site was bordered to its southern edge by a strategic A-road carrying significant HGV traffic, and the entire site lay within an Air Quality Management Area (AQMA) that the local authority had declared in respect of exceedances of the annual mean nitrogen dioxide objective.

The scheme was strategically important to the housebuilder's regional pipeline and had outline allocation in the emerging Local Plan. The application was for full planning permission, with a submission deadline driven by the housebuilder's own land-acquisition timeline.

The Challenge

Officers had flagged early concerns that introducing 250 new dwellings into an AQMA would expose future residents to elevated NO2 concentrations — a long-standing tension in planning law where a development is policy-compliant in principle but raises legitimate concerns about the suitability of the receiving environment for new sensitive uses. A first-pass screening indicated that, on the southern facade closest to the A-road, the predicted annual mean NO2 concentration was likely to exceed the 40 µg/m³ objective. Without a robust technical answer, the application was at real risk of either refusal or onerous conditions that would compromise the scheme's deliverability.

The local authority had also been clear that they expected detailed dispersion modelling rather than a screening-level assessment, and that they would scrutinise the assumptions built into any future-year projections.

Our Approach

We commissioned detailed ADMS-Roads dispersion modelling of the local highway network, building the model on a calibrated baseline using the latest available local diffusion tube data from the council's monitoring network. Verification factors were derived following the Defra LAQM TG(22) procedure to ensure the model was correctly anchored against measured concentrations at nearby receptor locations.

For the future-year assessment, we incorporated the appropriate NO2 declining-trend factors from the Defra background mapping tools, recognising that fleet turnover and the gradual electrification of the vehicle parc means that NO2 concentrations are projected to fall over the development's build-out period. Sensitivity testing was carried out to demonstrate that the conclusions held up even under more conservative trend assumptions.

We then worked with the architect and the masterplanner to test three mitigation options. The first was a building setback — pulling the southernmost block 4m further from the highway boundary than the indicative layout had shown. The second was a mechanical ventilation strategy with NO2-rated F7-grade filtration on intakes serving the units closest to the A-road. The third was a layout amendment placing habitable rooms (living rooms and bedrooms) on the leeward elevation, with kitchens, bathrooms and circulation on the road-facing side.

The final assessment combined all three measures and demonstrated that, with mitigation in place, the annual mean NO2 objective would be achieved at all building facades and at all garden amenity locations. We also calculated the human exposure-weighted impact descriptor following IAQM guidance, which we used to support a finding that the residual effect on local air quality was not significant.

The Outcome

The application was reported to planning committee with an officer recommendation for approval. A resolution to grant was secured. The Section 106 agreement included an air quality condition requiring post-completion verification monitoring, and the design mitigation was secured via planning condition. No appeal was needed and the scheme proceeded to the discharge-of-conditions stage.