Light spill onto bat commuting and foraging routes

Lighting and bats in planning is not an amenity question. Artificial light falling on a roost access point, a commuting route or a foraging area can amount to deliberate disturbance of a European protected species, and it can render a resting place functionally unusable even though nothing is physically damaged. That puts a lighting layout inside the same legal framework as a demolition method statement, and it is why an ecologist should be looking at the isolux plot before it goes anywhere near a planning submission.

Most schemes get this wrong in the same way. The ecology report says lighting will be sensitively designed. The lighting drawing, prepared separately, shows column heads at 6 metres along a hedge line.

Why lighting counts as an ecological constraint

The Conservation of Habitats and Species Regulations 2017 make it an offence to deliberately disturb bats in a way likely to affect their ability to survive, breed, rear young or, in the case of hibernation and migration, to hibernate or migrate. They also protect breeding sites and resting places from damage or destruction, and the courts and regulators have treated functional loss as capable of engaging that protection.

Light does that in three ways. It delays or prevents emergence from a roost, which shortens the foraging window at the most valuable part of the night. It creates a barrier across a linear feature that light-averse species will not cross, severing a commuting route. And it concentrates insect prey around lamps, which benefits a small number of fast-flying species and disadvantages the rest.

The joint guidance published by the Bat Conservation Trust and the Institution of Lighting Professionals is the document that authorities work from, and it is the reference an ecologist will expect to see cited.

Species sensitivity and why it changes the answer

Bats are not uniformly light-averse, and treating them as one group produces either an unbuildable lighting scheme or a useless one.

Horseshoe bats, long-eared bats and the Myotis group are strongly light-averse. They fly slowly, hug vegetation and abandon routes that become lit. Where these species are present, the mitigation has to be spatial: keep the light off the feature entirely.

Pipistrelles and noctules tolerate more, and will feed on insects drawn to lamps. That is not a reason to relax, because their roost access points remain sensitive and the feeding behaviour draws them across roads.

So the survey has to identify species and behaviour rather than presence alone. A transect showing which hedge is used for commuting, at what time and by which species, is what turns a generic lighting condition into a specific one. That evidence comes from a protected species survey carried out by a suitably licensed ecologist, and for bats a licensed professional is legally required rather than merely recommended.

Lux levels used in practice and where they come from

The figure most often written into conditions is a very low illuminance on the feature itself, commonly expressed as staying below 1 lux, with some authorities and some ecologists asking for less where highly sensitive species are present. Check the current edition of the joint guidance rather than quoting a figure from an older report, because the advice has been revised.

Two points about how the figure is applied. It is measured on the feature, which usually means a vertical or near-vertical plane at the face of a hedge or a tree line, not the horizontal illuminance on the ground that a lighting design normally reports. Ask the lighting designer for the right plane.

And it is a maximum rather than an average. A route that averages 0.8 lux with a 4 lux hotspot where a column sits close to the hedge is not compliant in any meaningful sense, and it is the hotspot that the bats respond to.

Designing dark space into a layout

The measures that work are ordinary lighting design decisions applied with a different objective.

• Position columns away from linear features rather than along them, and accept a slightly less even distribution across the carriageway.
• Lower mounting heights, which reduces spill even though it increases the number of luminaires.
• Full cut-off optics with no upward component, plus cowls, shields and louvres on the side facing the feature.
• Warm spectrum sources, avoiding the blue-rich white that attracts insects and disturbs light-averse species.
• Part-night dimming or switching through the hours of peak bat activity, with presence detection where a route must remain usable.
• Planting or fencing used as a physical screen where geometry alone will not deliver the reduction.

Continuity matters more than any single measure. A dark route with one lit gap is not a dark route, and the way to design an unbroken corridor across a site is set out in designing a dark corridor for bats across a site.

Residential schemes carry an extra problem that nobody has really solved. Occupiers fit their own security lights, and a condition binding the developer does nothing about a homeowner's floodlight three years later. Where the corridor matters, the honest answer is to put it in communal land under a management company rather than along private rear boundaries.

Coordinating the lighting and ecology reports

Sequence the work. Ecology survey first, then a constraints plan showing the features to be kept dark, then the lighting design, then an ecological review of the isolux output, then the submission.

The lighting assessment should include the environmental zone classification, the luminaire schedule with spectrum and optics, the isolux contours on the plane the ecologist needs, and a statement from the ecologist confirming the design meets the constraint. That last document is what an authority actually wants and is the one most often missing.

Where obtrusive light also has to be assessed for residential amenity, the environmental zone framework used for that work sits alongside the ecological assessment rather than replacing it. The two have different metrics and different thresholds, and a scheme can pass one and fail the other.

Conditions and verification after installation

Expect a condition requiring the lighting scheme to be approved before installation and implemented as approved, with no additional external lighting without consent.

Push for two further elements. A post-installation verification survey, measuring actual illuminance at the identified features once the scheme is energised and commissioned, with a mechanism for correction where readings exceed the limit. And a maintenance requirement, because replacement luminaires fitted by a contractor in year seven are almost always the wrong specification.

Verification is where schemes fail quietly. A design that predicts 0.6 lux at the hedge can deliver three times that once tree cover has been reduced or a column has been repositioned to avoid a service run. Measure it, and write the measurement into the discharge of the condition.

Enter your postcode on PlanScope to see which ecology and lighting documents your scheme is likely to need.

This is general guidance, not legal advice. Bat survey work requires a suitably licensed ecologist, and councils expect a suitably qualified author for lighting assessment work. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.