Lighting, bats and keeping a dark corridor
Bats avoid lit flight lines, so a scheme that lights a hedgerow has severed a commuting route whether or not anyone measured it. The design fixes are mostly geometric.
A bat sensitive lighting scheme keeps the illuminance on retained hedgerows, treelines and watercourses close to the existing dark condition, uses warm sources at or below 2700K, and relies on optical control rather than on dimming to get there. Most slower-flying British bats, including the horseshoe species, Myotis species and brown long-eared, avoid lit areas, and a line of light across a commuting route works as a barrier even where the habitat itself is untouched.
That is the mechanism an ecologist will describe and a lighting engineer has to design against. The two documents have to be produced together, because a scheme designed and then checked is almost always a scheme that has to be redesigned.
Why light severs a commuting route
Bats navigate between a roost and their foraging areas along linear features. The route is habitual and often used by the same individuals for many years. Light introduced along it does two things: it exposes bats to predation risk, which the light-averse species respond to by avoiding the area, and it concentrates insect prey around the luminaire, which draws some fast-flying species in and leaves the rest with less food elsewhere.
The consequence in planning terms is that lighting can constitute disturbance, and where a European protected species is affected it engages the Conservation of Habitats and Species Regulations 2017. Deliberate disturbance of a bat, and damage or destruction of a breeding site or resting place, are offences, with a licence from Natural England the route where they cannot be avoided.
Surveys and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the 2017 Regulations. No drafting tool substitutes for that.
Illuminance targets at hedgerows and water
The design target for a retained commuting feature is to keep added light to a level that does not materially change the dark condition at the feature. Practitioners commonly work to a very low maximum vertical illuminance at the hedgerow face, with the figure set by the ecologist for the species present rather than taken from a table.
That matters because the right number differs. A site with a horseshoe bat roost nearby will be given a tighter target than a site used by pipistrelles, and the ecologist should state the target, the receptor and the justification in the ecology report so the lighting designer has something to design to.
What the lighting report then shows is a vertical illuminance grid at the face of the feature, at the height the bats fly, with isolux contours. Horizontal illuminance on the ground is the wrong metric for this purpose and is the most common error in submitted schemes.
Watercourses need the same treatment, with the measurement plane at the water surface and along both banks. Bridges and culverts used as crossing points need checking separately, because a luminaire on an approach road can light the whole underside.
Colour temperature and spectral content
Short wavelength light, meaning blue and ultraviolet, attracts insects most strongly and scatters most in the atmosphere. Reducing it helps bats, insects and the night sky at once.
Specify a correlated colour temperature at or below 2700K, and lower where the ecologist asks for it. Amber and narrow-band amber products at around 1800K to 2200K are available and are the usual answer immediately adjacent to a sensitive feature.
Ask the manufacturer for the spectral power distribution rather than accepting the headline colour temperature, because two products at the same nominal figure can differ in their blue content. Avoid any source with significant ultraviolet output.
Lighting for people has a floor. Where a route must be lit for safety, the resolution is usually to move the route rather than to compromise on the spectrum.
Optics, shields and mounting height
The controllable variables are where the luminaire sits, how high, which way it points and what it is allowed to emit above the horizontal.
Design rules that work in practice:
- Keep upward light ratio at zero, with flat glass luminaires mounted horizontally and no tilt.
- Use the lowest mounting height that achieves the task, since a lower column gives a steeper cut-off towards the boundary.
- Specify asymmetric distributions that throw light into the site and away from the sensitive edge.
- Add louvres, baffles or rear shields where the geometry alone cannot achieve the target, and show them on the schedule.
- Set luminaires back from the feature, because distance is the cheapest attenuation available.
- Avoid floodlighting of elevations facing a retained hedgerow or a known roost access point.
Then model it. An isolux plot from lighting software, with the retained features drawn on it, is the evidence an ecologist can sign off against and an officer can condition.
Part night operation and controls
Switching off reduces exposure, and it is worth having, but it should not be the primary mitigation. Bat activity peaks in the hours after sunset, which is the same period when a car park or an estate road is most used.
Where part night operation is proposed, state the hours precisely and the control method. A curfew from midnight to 05:30 helps little if the sensitive period is the two hours after dusk in June. Dimming profiles that reduce output to 25 or 50 per cent during the peak activity period are often more useful than a full switch-off at midnight.
Presence detection on amenity lighting is useful on low-traffic routes and counterproductive on busy ones, where it simply stays on.
Record the control strategy in a way that can be conditioned and checked: a schedule of luminaires, their control group, their dimming profile and their hours.
Coordinating the lighting and ecology reports
The two reports should cross-refer explicitly. The ecology report identifies the receptors, the species, the roost and the commuting routes, and sets the illuminance target for each feature. The Lighting Assessment then shows the proposed scheme against those targets with modelled results, and references the ILP guidance on obtrusive light and the environmental zone that applies to the site.
Where the site is in environmental zone E0 or E1, the obtrusive light limits are tight and align reasonably well with the ecological requirement. In E3 or E4 the general limits are much looser than bats need, so the ecological target governs and should be stated as doing so.
A Preliminary Ecological Appraisal establishes whether bats are present at all, and where they are, a bat survey following the bat survey route with emergence and re-entry work between May and September is the evidence base. Scheduling matters: a scheme conceived in October cannot have emergence survey data before the following May.
Conditions and post installation checks
Expect a condition requiring the lighting scheme to be installed as approved and retained, and a further condition requiring no additional external lighting without consent. Both are reasonable.
What is worth offering voluntarily is a post-installation verification: a commissioned light survey measuring vertical illuminance at stated points along the retained features within a set period of first use, reported to the authority, with a remedial mechanism if the measured values exceed the target.
That turns a modelled promise into a checked outcome, and it is the single thing that most improves the credibility of a lighting chapter. It also protects the applicant, because a verified installation is a defence against later complaints.
Browse all 66 PlanScope report types and start a draft.
This is general guidance rather than 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. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
Which bats are most affected by lighting?
The slower-flying, light-averse species: horseshoe bats, Myotis species such as Natterer's and Daubenton's, and brown long-eared bats. Some faster species including common pipistrelle will feed around lit areas, which draws insects away from darker parts of the site. Both effects matter, so the assessment should name the species recorded rather than generalise.
What colour temperature should external lighting use near bats?
At or below 2700K as a general rule, and lower immediately adjacent to a sensitive feature, where amber products at roughly 1800K to 2200K are normal. Ask the manufacturer for the spectral power distribution rather than relying on the nominal figure, because products at the same colour temperature can differ considerably in blue content.
Is a horizontal lux plot enough evidence?
No. Bats fly past vertical features, so the relevant metric is vertical illuminance at the face of the hedgerow, treeline or water surface, at flight height. A horizontal plot on the ground tells the ecologist very little. Submitting the wrong plane is the most common reason a lighting chapter has to be redone.
Does switching lights off at midnight solve the problem?
Rarely on its own. Bat activity peaks in the hours just after sunset, so a midnight curfew misses the sensitive period. Dimming during the peak activity window, lower mounting heights, asymmetric optics and setting luminaires back from the feature do more. Treat part night operation as a supplement rather than the main measure.
Who has to prepare the bat survey?
A suitably licensed ecologist. Bat surveys and any mitigation licence are governed by the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017, and a licence from Natural England is required where disturbance cannot be avoided. A drafted report is a starting point for that professional rather than a substitute for them.
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.