Budgeting an air quality evidence base
Air quality budgets usually price the report and forget the monitoring. Where the money and the months actually go, from screening through to the damage cost calculation.
Most air quality budgets are wrong in the same way. They price the report and ignore the monitoring period, which is the item that decides whether you submit in March or in October. An air quality assessment for a scheme with no local monitoring data nearby can need several months of diffusion tube measurement before the modelling can even be verified, and that period cannot be compressed by paying more.
So the first question is not what an assessment costs. It is whether you need one at all, and if you do, whether you need monitoring to support it.
Screening before assessment
Screening decides the scope, and it is a short piece of work compared with what follows. Industry guidance published jointly by the air quality institutes sets indicative criteria based on the change in traffic flows a development would cause, with lower thresholds where the site sits within or adjacent to an air quality management area.
Three questions settle most schemes:
- Would the development materially change traffic flows on roads where people live, and by how much?
- Does it introduce new exposure, meaning homes or schools, into an area where concentrations are already close to the objectives?
- Does it include combustion plant, such as a biomass boiler or standby generators?
A scheme that answers no to all three usually needs a construction dust assessment and nothing more. Getting that documented early, ideally in a pre-application response, saves the cost of a full assessment nobody asked for.
Diffusion tubes and their duration
Where local monitoring is thin, nitrogen dioxide diffusion tubes are the standard way to establish baseline concentrations. They are inexpensive individually and the cost sits in the logistics: tube supply and laboratory analysis each month, a person visiting to change them, secure mounting at the right height and location, and bias adjustment using the national factors.
The programme point is the one to plan around. Tubes measure monthly, results need annualisation where fewer than twelve months are collected, and annualisation relies on a nearby continuous monitor with a full year of data. A three month campaign started in January and a three month campaign started in June give different answers, and the assessment has to account for that.
Six months is a common compromise. Twelve is better and rarely affordable in a development programme.
One saving worth chasing first: check whether the local authority already runs tubes near your site. Councils publish their monitoring locations and annual status reports, and a tube two streets away with three years of data can remove the need for a campaign altogether. That check takes an afternoon and occasionally saves six months.
Modelling and what it needs
Dispersion modelling converts traffic and emissions into predicted concentrations at receptors. The inputs are all external, and each one has a cost or a lead time:
- Traffic data, meaning annual average daily flows, speeds and fleet composition, normally taken from the transport assessment rather than surveyed separately.
- Emission factors from the national toolkit, which is updated periodically.
- Background concentration maps published nationally, mapped to the site.
- Meteorological data for a representative year, usually purchased.
- Local monitoring data for model verification, which is where the diffusion tubes earn their keep.
Verification is the step people underestimate. A model that has not been verified against local measurements will be challenged, and the verification factor can change the conclusion. Where no local data exists, this is the reason the monitoring campaign is unavoidable rather than optional.
Commission the modelling after the transport work rather than alongside it. A model run on superseded traffic figures has to be run again.
Receptor selection is the other place assessments go wrong. Receptors should sit where relevant exposure occurs, meaning facades of homes, schools and hospitals rather than points chosen because they produce comfortable numbers. An officer who spots a receptor placed at the back of a building when the bedrooms face the road will ask for the model to be rerun.
Construction dust assessment
Construction dust is assessed qualitatively under the recognised industry method. The site is scored for the risk arising from demolition, earthworks, construction and trackout, and that risk is set against the sensitivity of the surrounding area to produce a risk category and a mitigation package.
It is the least costly part of an air quality evidence base and the part most often required. On a small urban site with housing across the road it may be the entire assessment, and the output is a schedule of measures that ends up in the Construction Management Plan and in a condition.
Do not treat it as a formality. The mitigation listed is what the contractor is committed to, so a generic list copied from another site produces either a contractor who ignores it or a complaint the council has to act on.
Mitigation and the damage cost approach
Where a scheme has a residual effect, many authorities price mitigation using the damage cost approach, which applies published cost factors to the emissions a development is expected to generate. The result is a contribution towards measures in the authority's air quality action plan, secured by agreement.
Two practical points. The damage cost factors are published nationally and are revised, so use the current version and say which version you used. And mitigation delivered on site, such as electric vehicle charging, travel plan measures or boiler specification below the emissions standard, reduces the calculated figure, which means design decisions made early are worth money later.
Ask the environmental health officer how the authority applies the approach before the assessment is written. Practice differs, and a calculation run on the wrong basis has to be redone.
Reporting in an air quality management area
Inside a management area the assessment carries more weight and attracts more scrutiny, because the authority has already declared that objectives are not being met and has an action plan it is judged against.
Expect the assessment to address the action plan directly, to demonstrate that the development does not undermine it, and to justify the receptor locations chosen. Expect the environmental health officer to read the verification section properly. The annual mean objective for nitrogen dioxide is 40 micrograms per cubic metre, and a predicted concentration close to that figure at a proposed facade will be examined rather than accepted.
Programme and what to do first
Work backwards from the submission date.
- Screen at pre-application stage and get the scope agreed in writing.
- If monitoring is needed, order tubes and start immediately, because this is the long pole.
- Run the transport work in parallel, since the model depends on its outputs.
- Model and verify once traffic figures are fixed.
- Prepare the dust assessment last, as it depends on the construction methodology rather than on monitoring.
An air quality evidence base is normally described as professional recommended rather than legally required, and councils expect a suitably qualified author. The Air Quality Assessment page sets out what the report covers, and the same staged budgeting logic applies to acoustics, which we set out in budgeting an acoustic evidence base for a noisy site.
Drafting costs are the part you can fix. PlanScope pricing is a flat £50 per report or £150 per month on the Professional tier, and a draft can be sent for professional review before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Does my development need an air quality assessment?
It depends on whether it materially changes traffic flows on roads where people live, introduces new exposure into an area close to the objectives, or includes combustion plant. Industry guidance sets indicative screening criteria based on traffic changes, with lower thresholds inside or next to an air quality management area. Get the scope agreed in writing at pre-application stage.
How long does diffusion tube monitoring take?
Tubes measure monthly, and results from fewer than twelve months have to be annualised using data from a nearby continuous monitor. Six months is a common compromise in a development programme, and twelve gives the strongest result. The season matters, because a campaign run over winter and one run over summer produce different baselines.
Why does a model need verification?
Because dispersion models predict concentrations that must be checked against measurements taken locally before the outputs can be relied on. The verification factor derived from that comparison can change the conclusion of the assessment. Where no local monitoring data exists, this is why a monitoring campaign becomes unavoidable rather than optional.
What is the damage cost approach?
A method applying published cost factors to the emissions a development is expected to generate, producing a figure for mitigation secured through a planning obligation and spent on measures in the authority's air quality action plan. The factors are revised periodically, so use the current version and state which one. On-site mitigation reduces the calculated amount.
Is a construction dust assessment always needed?
Very often, and on smaller urban sites it may be the whole of the air quality work. It is a qualitative method scoring demolition, earthworks, construction and trackout against the sensitivity of the surrounding area, and it produces a mitigation package that goes into the construction management plan and into a planning condition.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.