Standby generators and the air quality assessment
Generators are assessed on the test regime: how often, how long, how many units at once. Describe it in the application or environmental health will ask.
A standby generator is assessed on how often you test it, not on how often the power fails. Emergency running is unpredictable and infrequent, so the air quality and noise case rests on the routine test regime: the weekly or monthly runs, their duration, the time of day they happen, and whether several units run together. Any standby generator planning air quality submission that describes the plant as emergency only, with no testing schedule, will come back from environmental health.
Developers of data centres, hospitals, laboratories and large office schemes meet this every time. The generators are a late addition to the design and an early objection in the consultation.
When a generator triggers assessment
Three things bring a generator into the assessment net: the size of the plant, its proximity to sensitive receptors, and whether the site sits in an Air Quality Management Area.
Expect an Air Quality Assessment where any of the following apply:
- The site is in or adjacent to an Air Quality Management Area
- Residential property, a school, a hospital or a care home is close to the proposed flue
- The scheme involves multiple units capable of running simultaneously
- Local policy requires an air quality assessment for the development type
- The flue discharge is at or below the height of nearby habitable windows
Councils usually expect a suitably qualified author for this work and professional review is strongly advised, since the assessment depends on dispersion modelling that an environmental health officer will scrutinise.
The pollutant of concern is nitrogen dioxide. The relevant benchmarks are the air quality objectives for England, which set an annual mean for nitrogen dioxide and a one-hour mean that may be exceeded only a limited number of times a year. Take the current figures from the Defra air quality pages rather than an older report.
Testing regimes and their emissions
A typical regime is a short no-load or light-load run weekly or monthly, with a longer load bank test annually or at longer intervals. The short runs matter for short-term concentrations and the long runs for cumulative emissions.
| Test type | Typical character | What it drives |
|---|---|---|
| Weekly or monthly short run | Minutes, low or no load | Short-term nitrogen dioxide peaks, noise complaints |
| Periodic load bank test | Hours, full load | Annual mass emissions, fuel use, sustained noise |
| Black start or resilience test | Multiple units together | Worst-case modelling scenario |
| Post-installation commissioning | One-off, full load | Compliance demonstration under the condition |
Model the worst case honestly, which means all units running at once under load. Promoters who model a single unit at light load get a condition limiting them to exactly that, and then cannot carry out the resilience testing their client requires.
Declare the regime in the application. A table of test type, frequency, duration, number of units and time of day is a short piece of work that prevents months of argument.
Dispersion modelling and stack height
The assessment runs a dispersion model using local meteorological data, building downwash effects and the receptor locations, then reports concentrations at each receptor against the objectives. The output that usually changes the design is the required stack height.
Stack height determination is iterative. A flue terminating too low sits inside the recirculation zone of its own building and delivers the exhaust straight back to the facade, so the model drives the flue up until the concentrations work. On a roof shared with heat pumps, extract plant and photovoltaics, that height has consequences for massing and for the townscape views.
Three design implications worth anticipating:
- The flue may need to rise above the parapet by more than the architecture expects
- Flue position is constrained by escape routes, plant access and intake locations
- Where the flue cannot go high enough, the answer is fewer units, lower emission engines, or abatement
Selective catalytic reduction and particulate filters exist and are used, but they add plant space, maintenance and cost, so test the stack height solution first.
Noise during testing
The noise case is a BS 4142:2014+A1:2019 exercise. The rating level of the generator during testing, with any correction for tonal or impulsive character, is compared against the measured background at the nearest sensitive receptor.
Generator noise has two characteristics that attract corrections: a strong low frequency component from the engine, and an impulsive start-up. Low frequency content travels and is poorly attenuated by lightweight screens, so acoustic enclosures, exhaust silencers and intake attenuators carry the mitigation rather than barriers.
A Noise Impact Assessment should model the full test scenario, including multiple units, and should state the time of day assumed. Daytime testing is far easier to justify than evening or weekend testing, and a condition restricting test hours is normal and generally workable, provided you asked for hours that suit the maintenance contract.
Fuel storage and bunding
Diesel storage brings its own requirements. Oil storage on a non-domestic site must be in a container with secondary containment, and the standard expectation is bunding with a capacity of at least 110 per cent of the tank it serves, under the oil storage regulations.
For planning purposes, the submission should show tank location and capacity, the bund arrangement, the fill point and spill containment, and any separation distance from drainage, watercourses or a source protection zone. Where the tank is above ground and visible, treat it as a design element, because an unscreened bunded tank on a prominent elevation draws objections of its own.
Where the site also carries battery storage, the siting and separation requirements interact, and battery storage alongside a solar scheme covers that interaction.
Permitting thresholds alongside planning
Planning permission is not the only consent. Generators above a thermal input threshold fall within the medium combustion plant and specified generator regime under the Environmental Permitting Regulations 2016, administered by the Environment Agency or the local authority depending on the plant.
The regime allows limited running for testing, commonly cited as up to 50 hours a year for emergency plant, which is the allowance that keeps most standby installations outside the emission limit requirements. Both the thresholds and the testing allowance have been amended, so confirm the current position with the regulator before fixing the test regime, and state in the planning documents which permitting route you expect to follow.
A permit and a planning condition can conflict. Where the planning condition limits testing to hours that the permit regime treats differently, you end up negotiating twice, so align them in the application.
Conditions commonly imposed
Expect some or all of:
- A limit on testing hours, days and times, often excluding evenings, Sundays and bank holidays
- A cap on the number of units tested simultaneously
- A noise limit expressed as a rating level at a defined assessment position shown on a drawing
- Flue height and position in accordance with the approved drawings
- Post-installation commissioning test results submitted and approved before first use
- Details of fuel storage and bunding approved before installation
Agree the assessment position and the noise limit on a drawing at application stage. The alternative is a dispute at commissioning, with the building finished and the generators installed, and that is the worst possible moment to discover the limit cannot be met.
Enter your postcode on PlanScope to see which reports a scheme with standby plant is likely to need. 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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why does testing matter more than emergency running?
Because testing is predictable and regular, while power failures are not. The assessment needs a scenario it can model, and the weekly or monthly test runs plus periodic load bank tests provide it. An application describing the plant as emergency only, without a stated testing schedule, gives environmental health nothing to assess.
Which pollutant drives the assessment?
Nitrogen dioxide, assessed against the air quality objectives for England, which set an annual mean and a one-hour mean that may be exceeded only a limited number of times each year. Take the current figures from the Defra air quality pages rather than from an older report, and state the source in the assessment.
How is stack height determined?
Iteratively, through dispersion modelling. A flue terminating too low sits in the recirculation zone of its own building and returns exhaust to the facade, so the model raises the height until concentrations at the receptors meet the objectives. The result often exceeds what the architecture anticipated, which affects massing and townscape views.
Do generators need an environmental permit as well as permission?
Above a thermal input threshold they fall within the medium combustion plant and specified generator regime under the Environmental Permitting Regulations 2016. The regime allows limited testing hours for emergency plant, commonly cited as up to 50 hours a year. Confirm the current thresholds and allowance with the regulator before fixing the test regime.
What bunding is required for a diesel tank?
Secondary containment, with the standard expectation being a bund of at least 110 per cent of the capacity of the tank it serves under the oil storage regulations. The application should show tank location and capacity, the bund, the fill point, spill containment and separation from drainage, watercourses and any source protection zone.
Which conditions should I expect on a generator consent?
Restricted testing hours and days, a cap on simultaneous units, a noise rating level at a defined assessment position, flue height and position tied to the approved drawings, commissioning test results before first use, and approval of fuel storage details. Agree the assessment position on a drawing at application stage.
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.