Lighting design for bats: guidance, lux levels and drawings

Lighting design for bats in planning is about keeping specific features dark, not about dimming the whole scheme. Bats navigate along linear features, hedgerows, tree lines, watercourses and woodland edges, and light across those features acts as a barrier. Several UK species avoid lit routes almost entirely, so a well-lit path along a hedgerow can sever a commuting route as effectively as removing the hedge.

That is why a lighting strategy is now a standard condition on schemes where a bat roost or a commuting route has been identified, and why the ecologist wants to see isolux drawings rather than a schedule of fittings.

Why artificial light counts as an ecological impact

Deliberate disturbance of a European protected species is an offence under the Conservation of Habitats and Species Regulations 2017, and lighting a roost access point or a foraging route can amount to disturbance. That puts light in the same category as demolition for legal purposes, even though nothing is being knocked down.

Light also affects prey. Many nocturnal invertebrates are drawn to short wavelength light, which concentrates them around luminaires and away from the dark habitat that slower-flying species use. The effect on lesser horseshoe and brown long-eared bats is well documented in the guidance and is more pronounced than for the faster, more adaptable pipistrelles.

The starting point for design is therefore the survey, not the lighting layout. A Preliminary Ecological Appraisal will identify the features to protect, and a Bat Survey with emergence and re-entry work will show which access points and flight lines actually matter on this site.

Identifying commuting routes and foraging habitat

Ask the ecologist to mark three things on a site plan before the lighting consultant starts.

Roost access points, with the elevation and height of each. Commuting routes, drawn as lines, with a stated buffer either side. Foraging habitat, drawn as areas, typically hedgerow and tree canopy edges, ponds and species-rich grassland.

That plan becomes the constraint drawing for the lighting design. Without it the lighting consultant is guessing, and the guess will usually be wrong because the ecologically important part of a site is rarely the part that feels dark to a person walking through it.

Where the survey has identified a roost that will be lost or disturbed, the design has to sit alongside the mitigation licence application to Natural England, and the licensed ecologist leads on what the licence requires.

Target illuminance on sensitive features

The Institution of Lighting Professionals and the Bat Conservation Trust publish joint guidance on bats and artificial lighting, and it is what councils and ecologists work from. The principle in it is straightforward: keep light off the feature, and where some spill is unavoidable, keep it as low as achievable.

Maintained illuminance at the feature is the metric, measured vertically at the hedgerow or tree line rather than horizontally at ground level. Conditions commonly fix a maximum in the region of 1 lux at the sensitive feature, sometimes lower on sites with lesser horseshoe bats, and the design is expected to demonstrate compliance rather than assert it.

The design hierarchy that ecologists accept, in order:

1. Do not light the feature at all, and lay out paths and parking away from it.
2. Where a route must be lit, use the lowest column height and the narrowest distribution that meets the human safety requirement.
3. Shield the luminaire so no light is emitted above the horizontal and none reaches the feature.
4. Only then consider dimming and part-night operation as the remaining control.

Reversing that order, which is what happens when lighting is designed first and mitigated later, produces a scheme with a lot of accessories bolted onto fittings in the wrong places.

Luminaire selection, spectrum and shielding

Spectrum matters as much as level. Warm-toned LEDs with a correlated colour temperature at the lower end of the available range, with the blue and ultraviolet content minimised, are what the guidance points towards. Metal halide and mercury sources, and cool white LEDs, are the worst performers.

Specify the optical control rather than the wattage. Flat glass luminaires with zero upward light output, asymmetric or forward-throw optics, and accessories such as louvres, baffles and internal shields do the work. Column height should be as low as the lighting class allows, because a lower column throws less light sideways into a hedge.

Bollard lighting is often proposed as the bat-friendly answer and is worth checking rather than assuming. Poorly specified bollards throw light horizontally at exactly the height of a low hedge, which is the opposite of what is wanted.

Where the scheme also needs to address obtrusive light to neighbours, the environmental zones and limits in the Institution of Lighting Professionals guidance on obtrusive light apply, and a combined Lighting Assessment covering both amenity and ecology avoids two documents contradicting each other.

Isolux drawings and what the ecologist checks

An isolux drawing shows contour lines of predicted illuminance across the site, calculated from the luminaire photometric files and the proposed layout. It is the evidence, and a fitting schedule is not.

What the reviewing ecologist looks for:

• Contours at low values, including 0.5 and 1 lux, rather than only at the design level for the path
• Vertical illuminance at the sensitive features, not just horizontal at ground level
• The calculation grid extended across the hedgerow or tree line, not stopped at the site boundary
• Maintenance factor and mounting heights stated, with the luminaire type and optic identified
• Any light spill from windows, signage and headlights acknowledged, since those are often larger sources than the external lighting

That last point is the one most often missed. A glazed lobby facing a hedgerow can deliver more light to the feature than the whole external scheme, and no amount of careful column selection fixes it.

Curfews, dimming and control strategies

Part-night operation, where lighting is dimmed or switched between defined hours, is a useful secondary control and a weak primary one. Bats emerge around dusk, which is the busiest period for people as well, so a curfew starting at midnight protects the quietest part of the night and misses the emergence peak.

Where curfews are used, tie them to sunset and sunrise rather than clock times, so the control follows the season. Presence detection on footpaths, with a low standby level, works better than a fixed switching regime on most residential schemes.

Dark corridors need to be continuous to be useful. A hedgerow that is dark for 80 m and lit for 20 m is not a commuting route.

The same applies to phasing. On a scheme built out over four years, the temporary construction lighting, site compound floodlights and security lighting between phases can light a feature that the finished scheme carefully avoids. Where a condition covers only the permanent lighting, the construction phase needs picking up separately in the site management documents.

Securing the scheme by condition

Offer condition wording rather than waiting for it. A workable condition requires a lighting strategy, including a layout, luminaire specifications, photometric data and isolux contours at stated values, to be submitted and approved in writing before the relevant part of the development is occupied, with the approved scheme implemented and retained thereafter.

Two additions are worth making. First, a requirement that the strategy is prepared in consultation with a suitably qualified ecologist, which stops a value-engineered relighting later. Second, a prohibition on additional external lighting without further approval, because the lighting that damages a dark corridor is usually the security floodlight added by an occupier in year three.

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. Draft the ecology and lighting documents your scheme needs on PlanScope, then have a licensed ecologist review them.