Lighting Assessment: obtrusive light limits by zone

A Lighting Assessment has to prove three things at once: that the scheme delivers the illuminance the use actually needs, that obtrusive light stays inside the limits for the site's environmental zone, and that the residual effects on neighbours and on wildlife are acceptable. A planning assessment that only does the first is a lighting design with a cover page.

Councils across England work to the Institute of Lighting Professionals guidance on the reduction of obtrusive light, revised in 2021. Officers read the zone, the receptors and the control regime, in that order.

What the assessment must demonstrate

Set out the design intent before any numbers. What is being lit, why, at what times, and to what standard. A car park serving a shift-working employer at 22:00 is a different case from an ornamental lighting scheme on a frontage, and the justification for each is different.

Then work through the four obtrusive light controls separately: upward light ratio, vertical illuminance at windows of relevant properties, luminous intensity in the direction of potential observers, and, where a building or structure is floodlit, its average surface luminance. Reporting a single headline figure for the whole site is the commonest way to lose a reviewer's confidence.

Choosing and justifying the zone

The zone drives every limit, so justify it in a paragraph rather than asserting it in a table. Say whether there is street lighting, what the nearest designation is, how far the nearest lit development sits and what you saw on a night visit.

Where the council has stated a zone in a local plan policy or design guide, use theirs and say so. Where two zones meet, the tighter limits apply at the shared boundary, so a suburban site backing onto unlit countryside is assessed against the darker zone along that edge. Assessments that quietly adopt the more convenient zone are the ones objectors take apart first.

Vertical illuminance and glare at receptors

Choose receptors on merit. The relevant point is the window of a habitable room facing the source, at the height it actually sits, not a point on the boundary fence or a notional plane at ground level.

Present a table of receptors with the predicted vertical illuminance at each, pre-curfew and post-curfew, against the applicable limit. Do the same for luminous intensity, taking the worst-case viewing direction rather than the one that suits the design.

Two technical points reviewers check. First, whether the calculation used a maintenance factor and which one, because a scheme reported at maintained values only will understate the light on day one. Second, whether reflected light from large pale surfaces has been considered on tight urban sites, since it can matter more than the direct component.

Curfew hours and condition wording

Limits come in pre-curfew and post-curfew pairs, and the curfew is commonly set at 23:00 unless the council specifies otherwise. Post-curfew values are far tighter and in the darkest zones reduce to zero.

Write the control regime in terms a condition can adopt. Which luminaires switch off entirely at curfew, which dim and by how much, which stay on for safety and why, and how the switching is achieved. A condition that says "external lighting shall be reduced after 23:00" is unenforceable and everyone involved knows it.

Where a scheme relies on motion detection, state the detection zone, the run-on time and how the sensor prevents triggering from the public footpath. Councils have started conditioning that level of detail because early schemes were defeated by a passing fox.

Say what happens on the darkest mornings too. A scheme that switches everything off at 23:00 and back on at 06:00 needs to explain how staff arriving at 05:30 in December get from the car park to the door safely, because the alternative is an occupier who quietly overrides the timeclock in the first winter.

Isolux drawings and the luminaire schedule

The drawing set does most of the persuading.

1. A site plan with isolux contours at ground level, showing spill across boundaries.
2. Vertical illuminance results at each identified receptor window.
3. A luminaire schedule giving make, model, wattage, lumen output, optical distribution, colour temperature and any shielding accessory.
4. A layout showing mounting heights, positions and aiming angles.
5. The calculation parameters: grid spacing, maintenance factor, reflectance values used.

Give the file references and revisions, and keep them consistent with the drawings submitted elsewhere in the application. Where the scheme is being installed in phases, show the lit extent at each phase rather than the end state alone.

One omission recurs often enough to be worth naming: contour drawings that stop at the red line. Spill does not, and a plan that shows the contours running into the neighbouring garden is more convincing than one that appears to have been cropped.

Ecology-led lighting design

On any site with hedgerows, mature trees, a watercourse or a building suitable for roosting, lighting and ecology have to be designed together. Several bat species avoid lit areas, and lighting a commuting route can sever it as effectively as removing the hedge.

The design moves are consistent: keep light off linear features, lower the mounting height, use warmer colour temperatures at or below 2700 K, specify accessories that cut the beam at the boundary rather than relying on optics alone, and switch off what does not need to run all night.

Where bats are present or reasonably likely, a Protected Species Survey is needed and the work must be carried out by a suitably licensed ecologist. That is a legal requirement under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017, and an AI-drafted document is a starting point for that ecologist rather than a substitute for one.

Where assessments fail on assumptions rather than numbers

The calculations are usually right. The inputs are where things go wrong.

A receptor list that omits the flat above the shop opposite. A zone chosen from a postcode rather than a site visit. A curfew regime described in the report but absent from the drawings. Luminaires specified generically so the contractor substitutes a lower-cost fitting with a different distribution and nobody notices until the complaint arrives. Boundary treatment assumed to be a 2 m fence that planning permission does not actually include.

And the one that causes the most argument after occupation: a scheme assessed for the lighting shown on the drawings, on a site where the occupier later adds security floodlights that were never part of the application. Offering a condition that requires any additional external lighting to be approved is a sensible piece of self-protection for the applicant as much as for the neighbours.

A Lighting Assessment is work councils expect from a suitably qualified author, and professional review is strongly advised. Where plant noise is also in issue, environmental health usually reads it alongside the Noise Impact Assessment, so keep the two consistent on hours of operation. Lighting is sometimes confused with daylight and sunlight work, which answers a different question entirely and is explained in the article on VSC, NSL and ADF.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and 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 lighting professional before relying on it.