Daylight modelling outputs a case officer can check

A daylight and sunlight report arrives with 40 pages of appendix tables and a conclusion that the effects are acceptable. You have a week to form a view and you are not a modeller. The good news is that most of the weaknesses in these reports sit in the assumptions rather than in the arithmetic, and the assumptions are checkable by anyone who knows which questions to ask.

This daylight report checklist is written for case officers and planning consultants reviewing a submitted assessment, not for the people producing one. Work through it in order, because a failure early on makes everything downstream unreliable.

The model geometry and where it came from

Ask first what the model is built from. A competent report states its sources, and the difference between a measured survey and an assumption is the difference between a defensible result and a guess.

What should be stated:

• The topographical survey used, with its date and surveyor, and whether it includes the neighbouring buildings or only the site.
• How the neighbouring building envelopes were derived: measured survey, laser scan, photogrammetry, published drawings, or scaled from aerial imagery.
• How window positions and sizes on neighbouring properties were established, and whether they were measured from the ground, from photographs, or assumed.
• Whether the proposed scheme was modelled from the submitted drawings, and which revision of those drawings.
• Ground levels, and whether they are to ordnance datum or to a site datum.

Then check the drawings revision against the application drawings. An assessment run on an earlier massing is common where a scheme has moved during pre-application, and the report will not flag it for you.

One detail that repays attention: whether existing balconies, canopies, projecting bays and boundary walls on the neighbouring property are in the model. Leaving them out overstates the existing baseline, which makes the proposed loss look smaller in percentage terms.

Reflectances, transmittance and maintenance assumptions

Vertical sky component is a purely geometric measure and carries no material assumptions. Everything else does.

Daylight distribution and internal illuminance calculations depend on surface reflectances, and the standard values are 0.7 for the ceiling, 0.5 for walls and 0.2 for the floor. Where a report uses higher values, it should say why, and higher reflectances make a room look better lit than the default assumption would.

Glazing transmittance is the second variable. Clear double glazing transmits roughly two-thirds to three-quarters of visible light, and the figure drops for triple glazing, solar control coatings and heavily framed systems. A report that models glazing as a clear void, with no transmittance applied, is overstating performance.

The framing fraction is the third. Slender frames on a drawing rarely match what is actually installed, and on a window with a transom and mullions the glazed proportion can be well under the opening area.

Maintenance factors apply to dirt on glazing in urban environments. They are not always used, and their absence should be visible rather than silent.

Which neighbouring windows were included

The scope of the assessment is where most disputes begin. A report that assesses six of the nine affected properties has made a judgement, and that judgement should be stated and justified.

The usual screening device is the 25 degree test in the BRE guidance: where no part of the new development subtends an angle greater than 25 degrees to the horizontal, measured from the centre of the lowest relevant window, the effect on that window is unlikely to be noticeable. That is a legitimate screen, and it is a proper reason to exclude a property.

Three things to check about scope:

1. Is there a site plan keyed to the window and property references used in the tables? Without it, the appendix is unreadable.
2. Have properties been excluded by the 25 degree test with a drawing that demonstrates it, or simply left out?
3. Are non-residential receptors considered where daylight matters to them, such as schools, and are commercial premises correctly excluded where it does not?

The BRE guidance applies to windows serving habitable rooms. Bathrooms, WCs, hallways, landings and stores are not habitable, so their exclusion is correct, but the report has to say which room each window serves and how that was established. Where the modeller has never been inside, room use has been inferred, and that inference should be stated openly, which is the subject of modelling neighbouring rooms when access is refused.

Rooms served by more than one window

This is where reports most often mislead without intending to.

Vertical sky component is measured at each window. Daylight distribution, the no sky line, is a property of the room. A room with a large main window and a small secondary one can show a severe percentage loss at the secondary window while the room as a whole is barely affected, and a report that leads with the window figures overstates the harm. The reverse also occurs where a report presents only room-level results and hides a badly affected window.

Ask for both, presented together, room by room. Then ask how the room dimensions were established, because daylight distribution depends on room depth, and an assumed depth of 4.5m in a house that has been extended to 7m produces a meaningless answer.

Corner rooms and rooms with windows on two elevations need the same treatment. So do rooms that have been knocked through, which are common in terraced streets and which change the analysis entirely.

The baseline the comparison uses

The BRE guidance assesses the effect of a proposal against the existing situation. That is the primary comparison and it should be the one presented first.

Reports sometimes offer an additional comparison against a consented scheme, a permitted development fallback, or a notional compliant envelope. Those can be useful context, and they are legitimate as supplementary material, but they are not the primary test and should not be presented as though they were.

Check three things about any alternative baseline: whether the consent is extant and implementable, whether it is likely to be built, and whether the comparison is presented alongside the existing-condition results rather than instead of them.

Where the site is currently cleared or occupied by a low building, the existing baseline is generous and almost any development will show losses. Say so plainly in the officer report rather than treating the percentages as though they were failures of design.

The tables that let you check the conclusion

A report you can audit presents, for each assessed window and room:

Percentages retained are more informative than absolute changes, and the 0.8 threshold is the one to look for, as explained in the 0.8 relative test and when a loss is noticeable. Sunlight results only apply to windows facing within 90 degrees of due south, and a report that presents sunlight figures for north-facing windows has produced noise.

Look for a summary that counts the windows and rooms by outcome band rather than a narrative that says most results are acceptable. If the report does not contain that count, do it yourself from the appendix, because the distribution of failures matters more than the average.

Omissions that undermine a report

Five absences should prompt a request for further information rather than a judgement on the merits.

No floor plans for the assessed neighbouring properties, so room uses and depths cannot be checked. No site plan keyed to the references. No statement of the drawing revision assessed. No sunlight assessment for amenity areas where gardens or communal space are affected. And no assessment of the proposed scheme's own internal daylight, which is a separate question from the effect on neighbours and is governed by the illuminance-based approach described in what changed in BRE 209 for new interiors.

Finally, read the conclusion against the numbers. A conclusion that describes results as broadly in line with the guidance, sitting above a table with several windows retaining under half their existing value, is not a conclusion. Ask for it to be rewritten before the officer report is drafted.

PlanScope can produce a first draft of a Daylight and Sunlight Assessment structured to these headings, which a qualified daylight consultant should model, check and sign off before it is submitted. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.