Obtrusive light: curfew hours and intrusion limits

Obtrusive light in planning is judged against two numbers and a clock. The numbers are the light falling on a neighbour's window and the light escaping upwards from your luminaires. The clock is the curfew, usually 11pm, after which the limits tighten sharply and in the darkest zones drop to zero. Getting an obtrusive light curfew planning condition you can live with depends on establishing the right environmental zone at the outset, because every limit follows from it.

The framework everyone uses is the Institution of Lighting Professionals guidance on the reduction of obtrusive light, and national policy expects new lighting to avoid harm to amenity and to intrinsically dark landscapes.

Environmental zones and how a site is classified

The guidance sorts locations into five zones, from protected dark areas through to city centres with high night time activity.

Classification is the single most consequential judgement in a lighting assessment, and it is often done carelessly. A retail unit on the edge of a market town is not automatically E4 because it has a lit car park, and a farm shop two fields from a village is not E2 because the village has street lighting.

Agree the zone with the council's environmental health or lighting officer before the assessment is written. Where the site sits on a boundary between zones, the nearest sensitive receptors decide the answer, because the limits protect them rather than the site.

Light intrusion into windows

The main amenity limit is vertical illuminance at a neighbour's window, measured in the plane of the glazing. The guidance sets a pre curfew value and a much lower post curfew value for each zone, tightening as the zone gets darker: in the guidance's own figures the limits fall from 25 lux before curfew in E4 to 2 lux in E1, with post curfew values of 5 lux in E4, 2 lux in E3, 1 lux in E2 and zero in E1 and E0. Take the current figures from the published guidance, because editions change.

Zero means zero. In an E1 location, a lit sign or a car park light that throws any measurable light onto a bedroom window after curfew fails, and the only compliant answer is to switch it off or shield it completely.

Two practical points follow. First, the curfew time itself is set by the local authority, and where the authority has not defined one the guidance's default applies, so ask rather than assume. Second, the limit applies at the window of a relevant property, which includes windows of rooms people sleep in above a commercial unit as well as houses across the road.

Luminaire intensity and upward light

Two further controls sit alongside intrusion.

Luminaire intensity limits the brightness of the light source as seen from the direction of a receptor, which is what causes glare. A fitting can comply on illuminance at the window and still be unacceptable because it is a bright point in someone's field of view.

Upward light ratio limits the proportion of a luminaire's output emitted above the horizontal, which is what produces sky glow. In the darkest zones the permitted ratio is zero, which in practice means full cut off luminaires with flat glass, mounted level, with no uplighters and no tilt.

The design moves that satisfy both are consistent. Lower mounting heights with more fittings rather than fewer tall columns. Flat glass and a full cut off optic. Correct aiming, recorded on the drawing rather than left to the installer. Shields and louvres on the side facing receptors. Warmer colour temperatures, which read as less intrusive and matter for wildlife. And a dimming or part night profile so the site is not lit at full output at 3am for nobody.

Sky glow and dark landscapes

In a National Park or National Landscape the intrinsic darkness of the area is itself the thing being protected, and the assessment has to address the cumulative contribution your scheme makes rather than just the effect on the nearest house.

That changes what the report has to contain. Expect to describe the existing night time character, to show the extent of new lit surfaces, and to explain why each luminaire is needed. The strongest argument in a dark landscape is not a compliant calculation. It is a shorter lighting schedule.

Colour temperature and ecology come together here. Lighting on hedgerows, tree lines and watercourses affects bats, which use those features as commuting routes, and the joint guidance from the lighting profession and the bat conservation sector sets out how to keep lux levels on those features low. Where bats are present, the lighting design has to be developed with the ecologist rather than presented to them, and a Bat Survey by a licensed ecologist is the starting point. Surveys and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the habitats regulations, and an AI-assisted draft is a starting point for that professional rather than a substitute.

Measurement and reporting

If a complaint or an objection turns on a number, the measurement has to be defensible.

1. Measure vertical illuminance in the plane of the window at the centre of the glazing, at the height of the affected room.
2. Take readings with the lighting on and off, and report the difference, so that street lighting and other sources are excluded.
3. Record the date, time, weather, the meter model and its calibration date, and the exact position with a photograph.
4. Measure after curfew as well as before, because the post curfew limit is the one that fails.
5. Present the results against the zone limits rather than as raw figures.
6. Include an isolux contour plot over a site plan for the design stage assessment, with the site boundary and receptor windows marked.

A Lighting Assessment built around those headings is what councils expect, and they usually look for a suitably qualified author, with professional review strongly advised before submission. This is a different exercise from the daylight work described in what a case officer checks in a daylight report first, and confusing the two is common: a Daylight and Sunlight Assessment is about natural light reaching windows, and obtrusive light is about artificial light leaving your site.

Conditions that can actually be enforced

A condition requiring that lighting shall not cause nuisance to neighbouring occupiers is unenforceable, because nobody can say when it has been breached. The conditions that work are specific.

Approval of a lighting scheme before installation, comprising a luminaire schedule with makes, optics and output, mounting heights, aiming angles, and an isolux plot showing predicted illuminance at the boundary and at named windows.

A curfew time, with the luminaires that switch off and the ones that dim identified separately.

A post installation verification report, measured on site and submitted within a stated period, confirming the predicted levels were achieved.

That last one is the condition operators dislike and should welcome. It closes the issue. Without it, an environmental health complaint two years later reopens everything, and artificial light emitted from premises can be a statutory nuisance under the Environmental Protection Act 1990, with only a defined list of premises types exempt.

Where a scheme should simply switch off

Some lighting has no case after closing time. Car park lighting for a site that shuts at 6pm, decorative facade lighting, illuminated signage after trading hours, and sports pitch floodlighting after the curfew all fall into that group, and offering to switch them off is usually easier than defending them.

Security is the standard objection to switching off, and it is weaker than it sounds. A site lit all night is not necessarily more secure, and the alternative of low level lighting on detection, with cameras that work at low light levels, is often accepted where continuous lighting is not.

One honest uncertainty. The guidance figures and the classification advice are revised from time to time, so confirm the current edition and the council's own supplementary guidance before you design.

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 rather than legal advice, so agree the environmental zone and the curfew with your local planning authority before finalising a lighting design.