Designing dark corridors for bats around a development

A dark corridor is a continuous, unlit or very dimly lit route that lets bats move between a roost and their foraging grounds after dusk. Getting one right is a lighting design problem with a legal edge: under the Conservation of Habitats and Species Regulations 2017 it is an offence to deliberately disturb bats or to damage or destroy a breeding site or resting place, and lighting a commuting route can amount to disturbance even when no building work touches the roost. The design target most authorities now expect is a vertical illuminance at the sensitive feature kept as close to zero as the scheme allows, with the joint Bat Conservation Trust and Institution of Lighting Professionals guidance setting a working figure of around 1 lux or less and lower still for slow-flying species.

That figure is not a threshold you pass. It is the number your lighting engineer has to model and show on a plan.

Why lighting matters more than the roost plan suggests

Bats are not uniformly light-averse. Some species, including pipistrelles, will feed around lit surfaces where insects gather. Slow-flying species, notably horseshoes and the Myotis group, avoid lit gaps almost completely, and a single lit driveway across a hedgerow can sever a route that has been used for decades. The roost stays intact, the animals lose access to it.

The practical consequence at planning stage is that a lighting layout added late, after the landscape drawings are fixed, is the commonest reason an ecologist's mitigation strategy stops working. Column positions follow the road geometry, the road geometry follows the layout, and by then the hedgerow that was going to be your dark corridor has a 6 m column 2 m from it.

Identifying the routes worth protecting

Your ecologist identifies routes from the bat survey work: emergence and re-entry counts, transects, and static detector data that show where activity concentrates. The features are usually linear and already on the topographical survey. Treelines, hedgerows, woodland edges, watercourses, ditch lines, stone walls, sunken lanes.

Ask for three things in writing before you start the lighting layout:

1. A plan showing the routes to be kept dark, drawn as a corridor with a width, not a line.
2. The species assemblage, because the target illuminance and the acceptable spectrum follow from it.
3. The sensitivity of each route, so you know which one to defend if two conflict.

Where surveys have not been done in season, you cannot design this properly, and a preliminary roost assessment alone will not tell you where the routes are. Emergence surveys run roughly May to September, with the best results May to August.

Luminaire selection, spectrum and aiming

Warm spectrum first. Blue and ultraviolet content attracts insects and increases avoidance, so a correlated colour temperature at or below 2700 K is the usual specification, and some authorities ask for 2200 K adjacent to sensitive features. Specify luminaires with no upward light and a sharp cut-off, which for road and amenity lighting means flat-glass optics rather than bowl diffusers.

Then aim. A luminaire angled to throw light along a footway, mounted on the far side from the hedge, with a house-side shield, does far more for a dark corridor than any amount of specification language. Add accessories where the geometry is tight: louvres, cowls, back-shields and internal baffles all have published photometric effects, so they can be modelled rather than promised.

Control matters as much as hardware. Part-night dimming from a fixed time, or a curfew on non-adopted lighting, protects the hours that count. Movement sensors are useful on private drives and bin stores and are a poor solution on a route where a passing resident triggers the light at the worst moment.

Mounting height, shielding and spill

Lowering a column reduces spill but tightens the spacing, and tighter spacing puts more sources near the corridor. There is a real trade-off here, and the answer depends on the corridor width. Where the dark route sits along a boundary, a lower mounting height with strong back-shielding usually wins. Where the corridor is a watercourse in the middle of a site, keep the sources further away and accept taller columns.

Spacing also interacts with the road adoption standard. Where the highway authority requires lighting to a class that the corridor cannot tolerate, the answer is usually to move the adopted route rather than to argue the class down, and that is a layout decision taken months before the lighting drawing exists.

Bollards deserve a warning. They are often proposed as the sensitive option and they sit at the height bats fly, throwing light directly into the flight path. Low-level linear lighting recessed into steps or handrails, aimed down, performs much better.

Showing the target illuminance at the corridor

The submitted evidence is an isolux contour plot, in vertical illuminance, at the face of the feature, not horizontal illuminance at ground level. Vertical is what a flying animal experiences. Ask for contours at 0.5 lux and 1 lux, calculated with the maintenance factor stated, and ask for the calculation grid to extend across the full corridor width.

Two details get missed. Light from windows, shopfronts and vehicle headlights does not appear in a street lighting calculation, so the plot understates reality where the corridor runs beside a road or a glazed frontage. And the lighting design has to be the one that gets built, which means the approved drawing should be referenced in the condition. A Lighting Assessment prepared for obtrusive light under the Institution of Lighting Professionals guidance covers a different question, human amenity, using environmental zones. Both may be needed, and they can reach different conclusions about the same column.

Landscape design that does the work for you

Planting is the least expensive mitigation available. A thickened hedge on the lit side of a corridor, a bund, a wall, or simply moving a footpath 4 m creates a shadow that no luminaire specification can. Coordinate the Landscape Strategy with the lighting layout so that the dark route is built into the plan form rather than retrofitted.

Think about establishment time. A new hedge takes several seasons to screen anything, so the mitigation has to work in year one with built form or fencing, then improve as planting matures. Say that in the strategy, with the interim measure named.

Securing the scheme and proving it afterwards

Expect a condition requiring a detailed external lighting scheme to be approved before occupation, prepared in accordance with the current bat and lighting guidance, and retained thereafter. Good conditions reference the isolux plot and the target illuminance at the named features. Weak ones reference a guidance document alone and get argued about later.

Post-installation verification is becoming more common: a night-time measurement of vertical illuminance at agreed points, reported to the authority. It costs very little, it finds the aiming errors, and it protects you if a complaint follows. Where a European protected species licence from Natural England is involved, the lighting design forms part of the licence method statement, and a licensed ecologist is legally required for that work. A drafted document is a starting point for that professional rather than a substitute, and the Protected Species Survey page sets out who can prepare what.

One honest uncertainty: the evidence base on species-specific lux thresholds is still developing, and the joint guidance has been revised as it grows. Check the current edition rather than a figure quoted in an older report.

Enter your postcode on PlanScope to see which reports a site with bat interest is likely to need.

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.