Lighting design that protects bats and invertebrates

External lighting is an ecological impact in its own right, and on a site with bats it can be the thing that turns a straightforward application into a licensing problem. Lighting a roost access point can amount to disturbance of a European protected species under the Conservation of Habitats and Species Regulations 2017, whatever the lux level on the ground. Lighting a hedgerow that bats commute along can sever a flight line as effectively as removing the hedge.

The design answers are well established and they are mostly about restraint: fewer luminaires, lower mounting heights, warmer colour temperature, tight shielding and a curfew. The evidence the council wants is an isolux plan showing illuminance at the habitat features, not a promise to use sensitive lighting.

Why lighting is an ecological issue

Three mechanisms do the damage, and they need separating because the mitigation differs.

Roost avoidance is the most serious. Light at or near an access point delays or prevents emergence, which shortens the foraging night and, in a maternity roost, affects the survival of young. This is the mechanism most likely to constitute an offence.

Barrier effects operate along linear features. Most British bat species avoid crossing lit gaps, so a single lit section of hedge or a lit road crossing can cut a colony off from its foraging grounds. The colony does not have to be on your site for your lighting to cause the problem.

Prey depletion works through invertebrates. Short-wavelength light attracts night-flying insects, concentrating them around luminaires and removing them from the wider habitat, and the moths caught circling a lamp are the food supply for the bats that avoid it. That is why a lighting design can harm bats it never shines on.

Flight lines and dark corridors

The first job is to establish where the bats go, which comes out of the bat survey rather than out of the lighting design. Activity transects and static detector data show the commuting routes; roost locations come from emergence and re-entry work.

Then design the dark corridors before you design the lighting. A dark corridor is a continuous, unlit route from the roost to the foraging area, with a width and a vegetated structure that a bat will use. Splitting a corridor with a lit path is the failure mode.

Practical measures that hold up:

• Route footpaths and roads away from retained hedgerows, treelines and watercourses, and where they must run alongside, put the planting between the light and the corridor
• Keep luminaires off the boundary that carries the flight line, and accept a lower design illuminance on that edge
• Use the building itself as a shield, placing lighting on the courtyard side rather than the field side
• Retain and plant structural vegetation to a height that blocks spill, rather than relying on the luminaire optic alone

The guidance most councils' ecologists work from is the joint Bat Conservation Trust and Institution of Lighting Professionals guidance on bats and artificial lighting. It sets the expectation that illuminance at roost access points and along important commuting routes is kept as close to the natural baseline as possible, which in practice means designing to a very low target rather than to an amenity standard.

Luminaire choice, spectrum and shielding

Four specification decisions carry almost all the ecological effect, and none of them costs extra.

Colour temperature: specify warm white, at or below 2700 K, and avoid blue-rich sources. Short-wavelength content is what attracts insects and what bats respond to most strongly.

Upward light: specify zero upward light ratio at the installed tilt. A luminaire with a flat glass and a horizontal mounting achieves it; the same luminaire tilted five degrees does not.

Shielding: use cowls, louvres, baffles and internal shields to cut spill towards habitat features, and state the accessory by reference in the schedule. An accessory that appears in the design and not in the purchase order is a common and entirely avoidable failure.

Mounting height and distribution: lower columns with tighter optics light a smaller area with less spill than taller columns with wider optics. Where a scheme needs adoptable street lighting, this is the point at which the highway authority's standards and the ecological objective collide, and it needs resolving before submission rather than at the discharge of conditions.

Spill levels and how they are demonstrated

The council's ecologist wants numbers at specific points, and the only way to provide them is a lighting model.

Submit horizontal illuminance contours across the site and, critically, vertical illuminance at the habitat features: the roost access point, the hedgerow face, the tree line, the water surface. Vertical illuminance is what a bat emerging from a gap experiences, and a plan showing only horizontal contours on the ground can look compliant while the wall containing the roost is lit.

The model needs the luminaire photometric files, the mounting positions and heights, the tilt, any accessories, and the maintenance factor. State the reflectances used, because a model that assumes dark surfaces understates spill from light-coloured render or pale paving.

Add a plan showing the dark corridors as a hatched overlay, with the modelled illuminance along them. A single drawing that shows the corridor and the numbers together answers most of the ecologist's questions without correspondence.

Curfews and control systems

A curfew converts a lighting scheme from a permanent effect to a partial one, and it is often the measure that makes a scheme acceptable.

The usual form is a part-night regime: full output until a stated time, then dimming or switching off for the core of the night, with the switch-off period covering the emergence and return windows for the species present. Where security requires some lighting, passive infrared control with a short run-on time keeps the average condition dark.

Two cautions. Motion-activated lighting on a route bats use can be worse than a constant low level, because a sudden change in illuminance is a stronger deterrent than a steady dim glow. And any control regime needs a management responsibility behind it, so name in the strategy who holds the settings and how they are protected from being reset by a future occupier.

Evidence in a lighting strategy

A Lighting Assessment prepared for an ecological purpose has a different centre of gravity from one prepared for obtrusive light and amenity, though the environmental zone framework in the lighting institution's obtrusive light guidance is still the right reference for the amenity element.

Include the ecological baseline and the flight line evidence, the luminaire schedule with colour temperature and accessories, the isolux plans in horizontal and vertical planes, the dark corridor plan, the control and curfew regime, the management responsibility, and a short statement from the ecologist confirming that the design meets the mitigation requirement. The ecologist's sign-off is the part councils look for first, and it is often missing.

Where the site also has a Preliminary Ecological Appraisal recommending further survey work, make sure the lighting strategy is written against the final survey results rather than the appraisal, because a strategy based on assumed flight lines will need redoing.

Discharging a lighting condition first time

Most lighting conditions fail at discharge for the same three reasons: no vertical illuminance at the habitat features, accessories specified in the text but absent from the schedule, and a luminaire substitution made after tender.

Work the submission the other way round. Fix the luminaire, obtain its photometric file, model with that file, and submit the model, the schedule and the cut sheets together. Where a substitution is unavoidable, get the ecologist to confirm equivalence in writing and submit that with the discharge application rather than explaining it afterwards.

Then protect the design on site. Include the lighting specification in the contract, mark the accessories on the installation drawings, and carry out a night-time check with a light meter at the habitat features before practical completion.

To see which ecology and lighting reports a site is likely to need, enter the postcode on PlanScope and flag trees and protected species in the guided questions.

This is general guidance, not legal advice. Survey work and mitigation licensing for bats require a suitably licensed ecologist under the Conservation of Habitats and Species Regulations 2017, and PlanScope's draft is a starting point for that professional rather than a substitute. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.