Counting affected windows and stating the proportion

The proportion of windows affected daylight figure is the number a committee remembers, and it is the number most easily manipulated without anyone lying. A scheme can report that 92 per cent of assessed windows meet the guidance while three flats lose half their light, and the arithmetic will be correct. Officers have learned to look for that, so a results summary that survives scrutiny states the totals, the denominators and the outliers on the same page.

Why totals matter more than individual windows

Nobody at committee reads a 200-window appendix. What gets used is a paragraph in the officer report, and that paragraph will contain a proportion.

So the presentation decision is a planning decision. If you do not summarise the results, someone else will, usually an objector's consultant with a different denominator. Producing the summary yourself, with the method stated, is how you keep control of what gets quoted.

The summary also has to serve the balancing exercise. National policy supports making effective use of land, and a degree of daylight effect is accepted in dense urban contexts where the guidance itself says the numerical targets may need to be applied flexibly. That argument only works where the residual effects are described precisely enough for the decision-taker to weigh them against the benefits. A vague summary loses the flexibility argument rather than winning it.

Counting by window, room, dwelling and property

Four units of account are available and they give four different numbers. Report all four, and label each clearly.

The window is the unit for the vertical sky component and for annual probable sunlight hours. The room is the unit for daylight distribution, the no sky line test, and rooms are also what people live in. The dwelling is the unit that matters politically, because one affected room makes a flat an affected flat. The property or address is the unit that maps onto objections.

• Windows assessed, windows meeting the guidance, windows falling below it.
• Habitable rooms assessed, rooms meeting the guidance for distribution, rooms falling below.
• Dwellings with at least one room falling below the guidance, against total dwellings assessed.
• Addresses affected, with the number of rooms affected at each.

State the denominator every time. "Ninety-two per cent of windows" means nothing without the total, and a total that includes bathroom and stair windows is a diluted total. Say which rooms were treated as non-habitable and on what basis, because that decision moves the percentage more than any other single choice in the report.

Where several windows serve one room, say so and say how you treated them. Reporting a room as adequately lit because one of its three windows performs well, when the other two are heavily affected, is the kind of presentation that gets a report set aside.

Weighting a small absolute loss against a large one

The BRE guidance works through ratios and absolute values together, and reporting only one of them distorts the result.

A retained vertical sky component of 24 per cent against a baseline of 30 per cent is a ratio of 0.8, which is at the edge of the guidance and is a small real change. A retained value of 12 per cent against the same baseline is a ratio of 0.4, and the room will feel different. Both appear in a simple pass and fail count as one failure each.

So report the retained value, the baseline value and the ratio for every window that falls below the guidance. Then group the failures by the extent of the shortfall so the reader can see the shape of the distribution.

Practitioners commonly band the results into minor, moderate and major categories using ratio thresholds. Those bands are a presentational convention rather than part of the BRE guidance, and the honest thing is to say so in the report and to state the thresholds you used. Inspectors accept banded presentations. They react badly to bands presented as if they came from the guidance.

Sunlight needs the same treatment. Annual probable sunlight hours are assessed against an annual figure and a winter figure, and a window that meets the annual test while failing the winter test is a different case from one that fails both. Report both columns.

Avoiding a presentation that conceals outliers

Three techniques reliably hide the worst results, and all three are spotted.

Averaging. A mean retained ratio across 200 windows tells you nothing about the flat that loses three quarters of its light. Never lead with an average.

Aggregating across properties. Reporting that 94 per cent of rooms across a street meet the guidance conceals the fact that all the failures are in one building whose occupiers will object. Group by address.

Silent exclusions. Rooms omitted because they are lit by a window under a balcony, or because the internal layout was assumed, need to be listed with the reason. An exclusion the reader finds for themselves destroys the credibility of everything else.

Do the opposite deliberately. Put a short paragraph in the summary that names the worst affected property, states the number of rooms involved and the retained values, and explains what was done to reduce the effect. A Daylight and Sunlight Assessment that volunteers its worst result is read as reliable, and the rest of its numbers are then accepted.

Tables officers can reproduce in a report

Design the tables for the person writing the committee report, not for the appendix.

A window-level table needs: address, floor, room reference, room use, window reference, existing value, proposed value, ratio, and whether the guidance is met. A room-level table needs the same identifiers plus the existing and proposed no sky line areas and the ratio. Sort by ratio, worst first, so the officer can quote the top of the table.

Then provide a one-page summary table that the officer can paste into a report. Totals by unit of account, counts by band, the worst case named, and a footnote stating the method, the guidance edition and the baseline used.

The baseline deserves its own sentence. Assessing against the existing situation gives one answer, and assessing against the existing situation plus a consented but unbuilt scheme gives another, sometimes a much more favourable one. Both can be legitimate. Using the second without stating it plainly is not, and it is the point on which a well-prepared objector will focus.

Grouping by property and by street

Objections arrive by address, so organise the results the same way.

For each affected property give the number of habitable rooms assessed, the number affected, the worst retained ratio, and a one-line description of the effect in plain words. "Two of six habitable rooms fall below the guidance, the worst retaining 0.62 of its former vertical sky component, both rooms being bedrooms at first floor" is something a case officer can use directly.

Street-level grouping then shows the pattern. Where all the effects fall on one flank of a single building, say so, because it points to the massing change that would fix it and lets you explain why that change was or was not made. Where the effects are spread thinly across many properties, that is a different planning judgement and the summary should make it visible.

Add a plan. A site plan with each assessed property outlined and shaded by the worst ratio it experiences communicates more in one view than any table, and it is the drawing objectors and officers both use. The approach to setting expectations in constrained sites is covered in daylight in a dense urban block.

Summarising without distorting

The summary needs four elements and no more.

1. What was assessed: the properties, the number of windows and rooms, the rooms excluded and why.
2. The method and the guidance edition, with the baseline stated.
3. The results by unit of account, with denominators, and the distribution of shortfalls by extent.
4. The worst affected property named, with the mitigation considered.

Then stop. A summary that argues the planning case is a summary the officer will not trust, and the planning argument belongs in the planning statement where it can be tested against policy. Where the assessment also covers the internal daylight of the proposed units, keep that reporting separate from the neighbour assessment, because the criteria and the audience differ, as set out in daylight inside new flats compared with neighbours.

One honest uncertainty worth stating in any assessment. Internal layouts and room uses in neighbouring properties are frequently assumed from external observation, and a wrong assumption changes the result for that room. Say how the layouts were established, and where they were assumed, say that too. An Overshadowing Study covering amenity space should carry the same note about assumed garden use.

PlanScope drafts a Daylight and Sunlight Assessment structure from your project details and your own modelling results, with "TO BE CONFIRMED" flags where values are missing. 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, and councils expect daylight modelling to be prepared by a suitably qualified consultant.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your scheme is likely to need.