Obtrusive light assessment: report structure to GN01
A GN01 lighting assessment is checked parameter by parameter. Here is the reporting structure, from zone justification to post-installation verification.
A lighting assessment written to GN01 is judged on whether an environmental health officer can check it without redoing the calculations. The Institution of Lighting Professionals guidance note on the reduction of obtrusive light sets out four control parameters, sky glow, light intrusion into windows, luminaire intensity towards observers and building luminance, each with limits that vary by environmental zone and by whether the time is before or after curfew. A report that reaches conclusions on those four parameters, in that order, with the zone justified and the survey method stated, is a report that gets conditioned rather than queried.
Environmental zone selection and justification
Start here, because every limit in the assessment follows from the zone. The zones run from E0, for the darkest locations such as dark sky reserves, through E1 for intrinsically dark areas including National Parks and National Landscapes, E2 for rural low district brightness, E3 for suburban medium brightness, to E4 for high brightness town and city centres.
Assign the zone for the receptor rather than for the development. A depot in an E4 retail park that backs onto an E2 village edge is assessed against the receiving zone at the boundary, and reports that pick the more convenient of the two get sent back.
Justify the choice in two or three sentences with evidence: the local plan dark skies policy if there is one, the designation status of the area, the existing street lighting regime, and a measured baseline. Where the receptor sits on a boundary between zones, say so and assess against the stricter one, or explain properly why not.
Baseline light levels and survey method
A baseline survey is what separates an assessment from an assertion. Record the date, the time, the weather, the moon phase, the instrument make and model, its calibration date, the measurement heights and the exact positions with coordinates.
Measure vertical illuminance in the plane of the windows most likely to be affected, and horizontal illuminance at ground level where the concern is about spill onto habitat or a highway. Take readings with the existing lighting on and, where it can be arranged, with it off, because the difference is the number the officer will want.
Say what could not be measured and why. Access to a private garden may be refused, and a reasoned estimate from a comparable position, clearly labelled as such, carries more weight than a gap in the table.
Proposed lighting design description
This section is where most drafts are thin. Give the schedule of luminaires with the manufacturer's type reference, the light output, the correlated colour temperature, the optical distribution, the mounting height, the tilt angle and the mounting position on a drawing.
Colour temperature is worth stating in every case. Warmer sources at or below 3000K, and lower where bats or dark skies are in play, are increasingly written into conditions, and a design submitted at 4000K because that is what the supplier stocked tends to get refused on a rural site.
Include the isolux contour drawings, the calculation grid spacing, the maintenance factor used and the software and version. An assessment without a stated maintenance factor cannot be checked, because the same design gives different answers at different factors.
Where lighting affects a foraging corridor or a roost, cross-refer to the ecology work rather than repeating it. Guidance on bats and artificial lighting is a separate document with its own limits, and the bat survey should be the source of the buffer distances used.
Vertical illuminance at windows
Light intrusion is the parameter that generates complaints, so report it property by property, not as a site maximum.
For each affected window, give the vertical illuminance in lux at the window plane from the proposed installation alone, then the same figure with the existing lighting included, and compare both against the limits for the zone before and after curfew. The guidance sets different figures for each zone and a much tighter post-curfew value, so a single column of results without the pre and post distinction is incomplete.
State the window use. A bedroom window is the receptor that matters after curfew, and an assessment that reports the illuminance on a garage door beside it has answered the wrong question.
Luminaire intensity and sky glow
Luminaire intensity is measured as the intensity in candelas emitted in the potentially obtrusive direction, towards a stated observer position. Give the observer positions on a plan, and give the intensity for each luminaire that contributes, not just the worst.
Sky glow is controlled through the upward light ratio, the proportion of the luminaire flux emitted above the horizontal when mounted in its design position. Full cut-off optics mounted flat give a ratio of zero, and that is the answer to aim for in every zone below E4. Where any tilt is proposed, state the angle and recalculate, because a tilted flat glass luminaire stops being a zero upward ratio product.
Building luminance applies where facades, signs or floodlit elevations are proposed. Report the average luminance of the lit surface in candelas per square metre against the zone limit, and treat internally illuminated signage separately.
The numeric limits for each zone sit in the current edition of the guidance note. Quote the edition, state the figures you have applied in a short table of your own, and do not reproduce the guidance wholesale.
Curfew hours and control measures
Curfew is normally taken as 23:00 to 07:00 unless the council specifies otherwise, and the post-curfew limits are far tighter than the pre-curfew ones. That makes the control strategy part of the assessment rather than an afterthought.
Set out what happens at curfew in operational terms:
- which circuits switch off entirely
- which dim, and to what percentage of output
- which remain on for safety or security, with the justification for each
- how the control system is programmed, whether by time clock, photocell or building management system
- who holds responsibility for the settings after handover
Security lighting is the usual sticking point. Movement-activated luminaires with a short timed run and a tight detection zone are normally acceptable where permanent illumination is not, and saying that plainly in the report often removes the objection before it is made.
Conditions and post-installation verification
Offer the conditions. A Lighting Assessment that ends with a suggested condition wording gives the case officer something to lift into the decision notice, and it keeps control of the wording with the applicant.
A workable condition secures the approved luminaire schedule and mounting details, the tilt angles, the curfew regime and a post-installation verification report. That last item matters more than the rest. Commissioning surveys regularly find aiming angles that drifted during installation, and a verification condition catches them while the contractor is still on site.
Propose the verification method in the assessment: repeat measurements at the same positions, at the same heights, with the same instrument type, within a stated period after the installation is energised, reported to the council for approval. And propose a remedial mechanism, because a verification report with no obligation to correct a failure is a document rather than a control.
Where the scheme is sensitive enough that the council will want an independent view, the professional directory is the route to a reviewer with lighting competence. AI-assisted drafting can structure the report and hold the parameters in the right order, but the measurements, the calculations and the professional judgement have to come from a lighting practitioner. The full report index shows which other documents a lit scheme usually needs alongside it.
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. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Which environmental zone should I assess against?
The zone of the receptor, not the development. Where a lit site in a brighter zone adjoins a darker one, the boundary receptors are assessed against the darker zone. Justify the choice with the designation status, any dark skies policy in the local plan, the existing lighting regime and your baseline measurements.
What is the usual curfew period in a lighting condition?
Most councils apply 23:00 to 07:00 unless a local policy says otherwise. The obtrusive light limits tighten sharply after curfew, so the assessment has to report pre-curfew and post-curfew results separately and set out which circuits switch off, which dim and which stay on with a stated justification.
Why does the maintenance factor matter in a lighting report?
Because the same design produces different illuminance results at different maintenance factors, and a reviewer cannot reproduce the calculation without knowing the value used. State the factor, the calculation grid spacing, the software and version, and the mounting and tilt assumptions alongside the results.
How should colour temperature be reported?
State the correlated colour temperature for every luminaire type in the schedule. Warmer sources, often 3000K or below and lower still near bat habitat or dark sky areas, are commonly secured by condition. A design submitted at a cooler temperature because of supplier stock is a frequent reason for objection on rural sites.
What does a post-installation verification condition require?
Repeat measurements at the same positions and heights as the baseline, using an equivalent instrument, within a set period after the lighting is energised, reported to the council. Aiming angles often drift during installation, so the condition should also oblige the applicant to correct any failure found.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.