Lux limits at a bat roost access point

A lighting condition that promises "bat sensitive lighting" is unenforceable, and most reviewers know it. What makes a lux limit bat roost condition work is a number attached to a point on a drawing: a stated maximum vertical illuminance at an identified access point, calculated on a named drawing, with the luminaire schedule and aiming angles fixed. The joint guidance from the Bat Conservation Trust and the Institution of Lighting Professionals asks for illuminance at a roost access point to be as close to zero as achievable, and conditions in practice are commonly written at 1 lux or below.

The legal driver sits behind that. All bat species are European protected species, and regulation 43 of the Conservation of Habitats and Species Regulations 2017 makes it an offence to damage or destroy a breeding site or resting place, or to disturb bats in a way that affects their ability to use a roost. Lighting an access point can do both without anyone touching the building.

Why a general lighting statement is inadequate

A commitment to minimise light spill cannot be breached. That is the whole problem. An enforcement officer standing in the garden with a meter has nothing to compare a reading against, and a reviewer signing off the design has nothing to check.

The second failure is that general statements are written before the lighting layout exists. By the time a contractor procures luminaires, the only record of intent is a sentence in an ecological appraisal, and the specification that arrives is whatever the electrical subcontractor has in stock. Conditions requiring a scheme to be submitted and approved help, but only if they say what the scheme has to achieve.

So the useful output of the ecological work is not a policy paragraph. It is a coordinate, a plane, and a figure.

Setting a limit at a stated point

Three things have to be pinned down before a number means anything.

The location. Roost access points are specific: a gap under a ridge tile, a lifted lead flashing, a soffit joint, a gap behind a barge board. Number each one on a plan and an elevation, cross-referenced to the emergence survey that identified it.

The plane. Illuminance at a facade is measured vertically, in the plane of the opening and facing outwards, not horizontally on the ground below. Horizontal readings understate what a bat emerging into the light experiences, and a condition that fails to state the plane will be argued about.

The figure, and what it includes. State whether the limit is on maintained illuminance with the maintenance factor applied, whether existing lighting on and off site is included in the calculation, and whether reflected light from pale surfaces has been accounted for. A limit of 1 lux from new luminaires alone, on a site where a neighbouring car park already delivers 3 lux at the same point, achieves nothing.

Producing an isolux drawing that proves it

The drawing does the work. What a reviewer needs to see:

• A site plan with the calculation grid stated, and isolux contours down to at least the 0.5 lux line
• An elevation of each affected facade with vertical illuminance contours across it and the roost access points marked
• The luminaire schedule with manufacturer photometric file references, mounting heights, tilt angles and any shields or louvres
• The maintenance factor and the reflectance values assumed
• Existing lighting included or, where excluded, a note saying so and why
• A stated design illuminance at each numbered access point, and along any commuting route

Software output alone is not enough. Check the model against the drawings, because the commonest error is a luminaire modelled at the correct position but the wrong tilt, and a five-degree difference in aim changes a facade calculation materially.

Colour temperature and spectrum

Spectrum matters as much as quantity. Blue-rich white light and any ultraviolet component attract insects to the luminaire, which draws light-tolerant species to feed there and depletes prey along the dark routes that light-averse species depend on.

The practical specification is warm-toned LED at or below 2700 kelvin, with no ultraviolet output, and sources with a narrow spectral distribution where the lighting task allows. Older discharge sources with strong ultraviolet content should be replaced rather than shielded. Amber and narrow-band amber sources go further and are worth proposing where the lighting is for orientation rather than for a task requiring colour rendering.

Species sensitivity varies enough to affect the specification. Horseshoe bats and Myotis species are strongly light-averse and will abandon a lit access point. Some pipistrelle species are more opportunistic and will feed at lamps. A design calibrated to the least tolerant species present on the site is the defensible approach, and the species list comes from the survey rather than from an assumption.

Part night operation and controls

Part night regimes help where the lighting has a genuine curfew, and they are close to useless where it does not. Bats emerge around dusk and return before dawn, so switching a car park off at midnight leaves the sensitive period fully lit.

The most reliable outcome is usually a permanently unlit zone rather than a cleverly controlled lit one. Where controls are relied on, the condition should require a commissioning record and a post-installation verification survey, and our note on curfew hours and part night lighting conditions covers the drafting.

Dark corridors along flight lines

An access point limit on its own protects the doorway and not the journey. Bats follow linear features, and severing a hedgerow, tree line or watercourse with light can cut a colony off from its foraging ground as effectively as blocking the roost.

So the design should carry a stated maximum illuminance along the feature as well, measured at a defined height above ground and at intervals along its length. Achieving it usually means moving luminaires away from the feature, dropping mounting heights, adding back shields on the units nearest it, and accepting a lower lighting class on the adjacent footpath. Planting a new hedge as a light screen is a long-term measure and should not be presented as immediate mitigation. Our note on designing dark corridors for bats around a development sets out how the widths are established.

Illustrative example: a barn conversion with two confirmed access points in the west gable, a hedgerow commuting route along the northern boundary, and a new parking area between the two. The workable answer was bollard lighting on the access road only, no lighting on the north side at all, a back-shielded wall light at the entrance door aimed away from the gable, and a stated limit at each access point verified after installation. The figures are illustrative of the approach rather than a design for another site.

Wording it so a condition can secure it

Draft the condition the way you would want to read it as a reviewer two years later. The elements that make it work:

1. Reference to a numbered lighting drawing and luminaire schedule, both dated.
2. A stated maximum vertical illuminance, in lux, at each numbered roost access point, and along the identified commuting route.
3. A statement of the plane and height of measurement and whether existing lighting is included.
4. A requirement that the maximum correlated colour temperature does not exceed a stated value.
5. Installation before first use, and a verification survey by a suitably qualified person submitted within a set period.
6. No alteration to luminaire positions, aiming, output or control settings without prior written approval.

The survey evidence underpinning all of this has to come from a licensed ecologist. Emergence and re-entry surveys and any mitigation licence sit under the Wildlife and Countryside Act 1981 and the 2017 Regulations, and a bat survey is work a licensed ecologist must carry out. A Protected Species Survey drafted in advance is a starting point for that ecologist, not a substitute for the survey, and the lighting calculations themselves belong in a lighting assessment prepared by someone competent to produce them. Reviewers who take on sign-off can register through the professional directory.

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.