Designing a dark corridor for bats across a site

Lighting is the part of a scheme that turns an ecology chapter into a refusal. Bats are European protected species under the Conservation of Habitats and Species Regulations 2017 and the Wildlife and Countryside Act 1981, and lighting a roost access point or a commuting route can amount to disturbance or obstruction of access to a place of shelter. A dark corridor is the design response: a continuous unlit or very low lit route along the hedgerows, treelines and watercourses that bats use to move across a site.

The target comes from the ecologist, not from the lighting standard. That inversion is what most design teams get wrong.

Why the ecological requirement leads

The Bat Conservation Trust and the Institution of Lighting Professionals publish joint guidance on bats and artificial lighting which works to a maintained illuminance of 1 lux or below along commuting routes and at roost access points, and as close to zero as the scheme allows. Some species are far more light averse than others. Horseshoe bats and long-eared bats will abandon a route at levels that a pipistrelle tolerates, so the figure your ecologist sets depends on what the surveys found.

Obtrusive light guidance from the ILP, which gives environmental zones from E0 intrinsically dark through to E4 high district brightness, sits alongside this rather than replacing it. You can satisfy an E3 urban limit and still lose a bat corridor.

Finding the routes before you design the lighting

Commuting and foraging routes come out of the survey work: a preliminary roost assessment, then activity transects and static detector deployments across the emergence season, which runs from May to September with the best data from May to August. A Preliminary Ecological Appraisal identifies the features worth surveying, and the bat survey itself establishes where the animals actually go.

What the lighting designer needs from that work, in writing:

• A plan showing each commuting route and foraging area as a line or polygon, not a general note that bats were recorded.
• The species and the sensitivity band the ecologist is applying.
• The target illuminance, stated as maintained horizontal and vertical values, with the height the vertical value applies at.
• The dark buffer width required between any lit surface and the vegetation edge.
• Any seasonal or nightly curfew, for example lights off or dimmed from an hour after sunset.

Without the vertical illuminance target the design cannot be tested properly, because bats fly along the face of a hedge rather than over the mown grass beside it.

Luminaire selection and spectrum

Warm spectrum first. Correlated colour temperature at or below 2700K is the normal expectation near sensitive habitat, and 2200K amber where the corridor is important. Blue-rich white light attracts insects, which changes prey distribution and draws some species towards lit areas while excluding others.

Then optical control. Specify luminaires with a zero upward light ratio and tight distribution, and use shields, cowls, baffles or internal louvres where the spill needs cutting on one side. Avoid ultraviolet output entirely. Avoid bollards along a corridor edge, because their horizontal component throws light straight into the vegetation at exactly the height that matters.

Controls do real work here. Part-night operation, dimming profiles stepping down after a set hour, and presence detection on secondary routes all reduce the cumulative exposure, and they are easy to secure by condition.

Column height, spacing and aiming

Lower columns reduce spill distance but need closer spacing, which raises the number of sources. Taller columns with good optics can sometimes produce a cleaner cut-off. There is no universal answer, so test both.

Practical rules that hold up:

1. Keep the lit route physically separated from the corridor, with built form, a bund or planting between them where possible.
2. Aim luminaires away from the corridor and accept asymmetric distributions rather than symmetric ones.
3. Do not light footpaths that run directly alongside a hedgerow. Move the path, or accept that it is unlit and design the layout so it is not a desire line at night.
4. Treat car park lighting, building facade lighting, signage and residents' own porch lights as part of the scheme, because the condition will be judged on the whole site.
5. Check headlight spill from driveways and turning heads. It is light, and a planning officer who has seen it before will ask.

Testing the design against the requirement

The deliverable is an isolux contour plot over the site plan, showing maintained illuminance at ground level and vertical illuminance at the height the ecologist specified, typically at the vegetation face. Overlay the corridor polygon and demonstrate that the contour for the target value sits outside it.

Send that drawing to the ecologist before it goes in the application. A short written confirmation that the scheme meets the requirement is worth more to the case officer than any amount of narrative, and it heads off the common outcome where lighting is approved in principle and then fails at the discharge of conditions stage.

Where surveys and mitigation licensing are involved, a suitably licensed ecologist is legally required under the habitats regime, and a Protected Species Survey cannot be produced any other way. A PlanScope draft gives that professional a structured starting point covering policy, method and reporting, and nothing beyond that.

An illustrative example

Illustrative only: a 60 unit scheme on a former paddock with a mature hedgerow along the southern boundary, recorded as a commuting route for soprano pipistrelle and brown long-eared bats. The layout is revised so the estate road sits 15 metres north of the hedge with rear gardens between them. Column lighting uses 2200K luminaires with rear shields at 4 metre mounting height, dimmed to 30 per cent after midnight. The isolux plot shows the 1 lux vertical contour falling short of the hedge face by several metres, and the ecologist confirms the corridor stays functional.

The cost of that outcome was a layout change at sketch stage, not a lighting product.

Securing it by condition

Ask for the condition wording you want. A workable version requires the development to be carried out in accordance with a named lighting drawing and isolux plot, requires any subsequent external lighting to be approved in writing, and ties the dimming or curfew regime to the approved scheme. Vague wording about lighting being kept to a minimum is unenforceable and leaves the corridor exposed once residents move in.

One more point on enforcement. A condition that bites at the discharge stage only is weak, because the risk to a corridor is cumulative and arrives with occupation. Ask for wording that also controls lighting added later by a management company or by individual householders in rear gardens, and reference it in the estate management documents so purchasers know before they buy.

Construction is the period everyone forgets. Temporary site lighting during winter working can be brighter than anything in the finished scheme, so deal with it in the construction management documents alongside the noise controls covered in construction noise, BS 5228 and section 61 consent.

A Lighting Assessment drafted early, with the ecologist's targets written into it, is the difference between a condition you can discharge and one that stalls the site.

This is general guidance, not 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.