Lighting design for bats: lux limits and spill control

A bat sensitive lighting scheme starts from darkness and adds light only where it is needed. That is the opposite of how most external lighting is designed, which is why lighting conditions are the most frequently failed ecology condition on small residential and conversion schemes. The design target is to keep illuminance on roost access points, commuting routes and foraging habitat at or close to the existing baseline, and the figure applied in practice is around 1 lux or less on those features, with a lower target on the darkest corridors.

Check the current edition of the joint Bat Conservation Trust and Institution of Lighting Professionals guidance for the figures, because they have been revised and councils cite the version current at the time of the condition.

Why lighting ends up in a condition

All bat species in the United Kingdom are European protected species. Under the Conservation of Habitats and Species Regulations 2017 it is an offence to damage or destroy a breeding site or resting place, and to deliberately disturb bats in a way that affects their ability to survive, breed or rear young, or to use their roost.

Lighting a roost access point can obstruct access and amount to disturbance without anything being demolished. Lighting a hedgerow that a colony uses to travel between roost and feeding ground can sever that route, because many species will not cross an illuminated gap. Neither effect shows up on a drawing that only shows luminaire positions.

So the council's ecologist asks for the lighting to be controlled by condition, usually in the form of a scheme to be submitted and approved before installation and then maintained. Where a roost is present or likely, the survey work behind it has to come first, and bat emergence survey effort and roost suitability determines what the lighting design has to protect.

Target illuminance on sensitive features

Identify the features before you set any targets. They are normally the roost access point and the elevation around it, the flight line used on emergence, the hedgerow or treeline the bats commute along, and any retained foraging habitat such as a pond margin or a woodland edge.

For each feature, record the baseline. A measured baseline taken at night with a calibrated meter is far stronger evidence than an assumed figure, and on a rural site the baseline may be close to zero.

Then set the target. The convention is to hold maximum illuminance on the feature at or below about 1 lux, measured in the plane that matters, which for a roost access is vertical rather than horizontal. Dark corridors are often specified lower. Where the baseline already exceeds the target because of street lighting, the design aim becomes no net increase, and that should be stated rather than used as an excuse to add more.

Luminaire selection and shielding

The specification does most of the work, and it is short.

• Warm colour temperature, commonly 2700 kelvin or lower, with no ultraviolet component. Blue-rich white light attracts more insects and is avoided.
• Zero upward light output, with flat glass and a horizontal mounting position rather than a tilted luminaire.
• The lowest practicable mounting height, because height spreads light.
• Full cut-off optics, with cowls, louvres, baffles or shields where the beam would otherwise reach a feature.
• Narrow beam angles and asymmetric distributions aimed into the site rather than across it.
• The minimum number of luminaires that meets the amenity and safety requirement, and no decorative uplighting.

Two points architects raise. Bollards are not automatically better than columns, because a bollard at 1 metre throws light into a hedge base that a shielded column would miss. And wall-mounted downlighters on an elevation containing a roost access are the single most common problem, so move the access or move the luminaire.

Security lighting is the other recurring difficulty. A floodlight on a passive infrared sensor can be acceptable if the beam is shielded away from features and the timer is short, but a 150 watt unaimed floodlight on a barn conversion will not be approved where bats use the building.

What the isolux drawing must show

The contours have to cross the boundary. A drawing that stops at the red line tells the ecologist nothing about the hedge on the other side, and that hedge is often the commuting route.

Also show what happens with the lighting off. A simple plan marking which areas remain unlit lets the officer see the dark corridor as a designed feature rather than an absence.

Dark corridors as a design decision

A dark corridor is a continuous unlit route between roost and foraging habitat. It only works if it is continuous, which means it has to survive the whole scheme, including the parking layout, the bin store, the footpath lighting and any future phase.

Design it in early and protect it on the layout plan. Keep built form and lighting back from the hedge line by a stated distance, avoid crossing the corridor with a lit access road, and where a crossing is unavoidable use the lowest, most tightly shielded solution and accept a gap in lighting rather than a gap in the corridor.

The corridor also interacts with habitat accounting. Retained hedgerow and the buffer around it feed into the habitat plan used for the Biodiversity Net Gain Assessment, and the two documents should use the same drawing. Biodiversity net gain under the Environment Act 2021 expects the statutory metric to be completed by a competent person.

Curfews and controls

Part-night lighting is often the measure that makes a scheme acceptable. Typical controls are a curfew switching off or dimming communal and amenity lighting after a stated hour, short timers on sensor-operated units, and separate circuits so that safety lighting can stay on while decorative and amenity lighting goes off.

Offer the curfew yourself and state the hour. A condition drafted by the applicant, with the circuit diagram showing which luminaires it covers, is far easier to discharge than one drafted by an officer who does not know how the site is wired.

Discharging the condition

Submit the lighting scheme as one package: the isolux drawings, the luminaire schedule and data sheets, the ecologist's written assessment of the design against the targets, the curfew and control arrangements, and a maintenance and replacement commitment so that a failed luminaire is not replaced with whatever is on the van.

Then verify after installation. A post-installation night-time survey with measured readings at each sensitive feature, compared against the approved contours, closes the condition properly.

Survey work for bats and any mitigation licence legally require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the 2017 Regulations, and that applies to the lighting assessment where it forms part of a licence application. A Protected Species Survey drafted with PlanScope is a starting point for that professional rather than a substitute for their judgement.

And keep the lighting designer and the ecologist in the same conversation. Most failed schemes are two competent documents that were never read against each other.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope produces AI-assisted draft reports for planning applications, and reports should be reviewed before submission. For specialist technical reports, qualified professional input may be required. PlanScope does not guarantee planning approval.