Reading the results tables in a daylight and sunlight report
Vertical sky component, no sky line, annual probable sunlight hours and target illuminance measure different things. Here is how to read each column, and where a summary table can flatter a scheme.
A daylight and sunlight report is mostly tables, and the tables carry the argument. The numbers in a BRE 209 daylight results table are not pass or fail marks. They are comparisons against guide values that BRE itself describes as flexible, applied to a baseline that the assessor has chosen. Your job when the report lands is to work out what each column measures, which ratio the council will fix on, and where the presentation is doing quiet work for the scheme.
Most objections that succeed are not about the modelling. They are about a baseline room use that nobody checked, or a summary table that reports averages over a block when the harm sits in four flats on the second floor.
What each column actually measures
Four measures do almost all the work in a neighbouring-amenity assessment, and they answer different questions.
Vertical sky component is a measure taken outside the building, at the centre of the window on its outer face. It tells you how much of the sky dome that point can see, as a percentage of an unobstructed horizontal surface. It says nothing about the room behind the glass.
No sky line, reported as daylight distribution, is taken inside. It divides the working plane into the part that can see sky and the part that cannot, and compares the two before and after.
Annual probable sunlight hours counts direct sun rather than diffuse skylight, and applies only to windows within 90 degrees of due south. Interior daylight in the 2022 edition of BRE 209 is reported against target illuminance, following BS EN 17037, rather than the average daylight factor that older reports used.
| Measure | Taken where | Guide value in BRE 209 (2022) |
|---|---|---|
| Vertical sky component | Outside face of window | 27 per cent, or 0.8 times the former value |
| No sky line | Working plane inside the room | 0.8 times the former sky-visible area |
| Annual probable sunlight hours | Outside face of window | 25 per cent annual, 5 per cent winter |
| Target illuminance | Across the room's assessment area | 100, 150 and 200 lux by room use |
A table that mixes these without labelling which is which is the first warning sign.
Vertical sky component and the 27 per cent figure
Twenty-seven per cent is the number everyone quotes and the number most often misused. BRE 209 gives it as the point below which a window may start to look poorly lit, in a low-density setting. It was never a planning standard, and in a dense urban block a large share of existing windows already sit below it before anything is built.
So the absolute figure matters least in exactly the places where schemes are most contested. What the assessor should be reporting alongside it is the retained ratio: the proposed vertical sky component divided by the existing one. Below 0.8, BRE treats the loss as likely to be noticeable to the occupant.
Look for the baseline. If the existing condition has been modelled with a neighbouring building that has since been demolished, or with a consented but unbuilt scheme included, the ratio changes and the harm can disappear on paper. Ask which planning status the surrounding massing is in. Vertical sky component and when a departure is justified covers the reasoning you need when the number is low and the context explains it.
No sky line and where daylight lands in the room
Daylight distribution catches the effect vertical sky component misses. A window can retain most of its sky view while the back half of a deep room loses the sky entirely, and the no sky line contour is what shows that.
The working plane sits at 0.85 metres for residential rooms. The report should give the sky-visible area before and after, in square metres, and the ratio. Where it gives only a percentage, ask for the areas, because a small absolute loss in a small room produces an alarming ratio and a large loss in a large room can look mild.
Room use drives the result here more than anything else. A living room assessed as a bedroom, or a room with a second window that was omitted because the drawing was old, will produce a result that does not survive a site visit by the case officer.
Annual probable sunlight hours and the winter split
Sunlight is reported as two numbers, and both have to be read. A window should keep at least 25 per cent of annual probable sunlight hours and at least 5 per cent in the winter months from 21 September to 21 March. BRE treats a loss as noticeable where the proposed value falls below 0.8 times the former value, the absolute figures fall below those targets, and the annual reduction is more than four percentage points.
That three-part construction is routinely reported as if any one limb settles the matter. It does not. A window can drop below 25 per cent annually while retaining more than 0.8 of its former value, which is a different and much weaker complaint than one where all three limbs are breached.
The winter figure is the one that reaches committee members, because a north-facing living room with 2 per cent winter sunlight is easy to picture. Check whether the assessor tested every window within 90 degrees of due south or only a sample.
Ratios matter more than absolutes
In an urban context the retained ratio is the defensible measure and the absolute guide value is background. The London Plan expects daylight and sunlight to be assessed with reference to the local context rather than to suburban targets, and inspectors have taken the same approach in dense schemes. That is a policy judgement, not a modelling one, so the report should set out the character of the existing daylight environment before it reports a single percentage.
Where a report leads with absolutes in a high-density site, it is usually because the ratios are worse. Where it leads with ratios in a low-density site, the opposite. Read both.
Alternative targets, sometimes called local context targets, are legitimate when the comparator is stated and justified. They are not legitimate when the comparator is the scheme's own massing.
Rooms that serve more than one use
Open-plan kitchen, living and dining space is where target illuminance arguments get sharp. BS EN 17037 sets different target illuminances by room use, and the 2022 edition of BRE 209 carries them through: broadly 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, achieved over at least half the assessment area for at least half the daylight hours.
Apply the kitchen figure to a combined room and the scheme performs worse. Apply the living room figure and it performs better. Assessors differ, councils differ, and the report should say which convention it used and why rather than leaving you to reverse-engineer it from the results.
Circulation space, bathrooms and stores are not assessed. If a scheme's summary shows an unusually high compliance rate, check how many of the counted rooms are ones nobody was worried about.
Reading the summary table without being misled
Work through the summary in this order.
- Confirm the baseline massing and its planning status, including any consented schemes included or excluded.
- Check the room uses against the existing floor plans, not the proposed ones.
- Separate the windows that breach a guide value from those that breach a ratio, and count them.
- Find the worst-affected rooms and read their individual rows rather than the block average.
- Check whether amenity areas were tested for two hours of sun on 21 March over at least half their area.
- Ask what proportion of the tested rooms sit in a single building or on a single elevation.
Averages across a whole street hide concentrated harm, and concentrated harm on a handful of neighbouring flats is what a refusal tends to be written around. The Overshadowing Study covers the amenity-area side of that test, and shadow plots and dates in an overshadowing study explains how the plots should be dated.
One honest uncertainty: BRE 209 is guidance, not a standard with a compliance threshold, and different inspectors weigh a 0.75 retained ratio very differently depending on the housing need in front of them. Anyone who tells you a number decides the case has not read many decision letters. Daylight in a dense urban block: realistic targets sets out where the flexibility lies.
If you want a draft Daylight and Sunlight Assessment structured to the right sections before your consultant runs the modelling, start from the report page and add the numbers once they exist.
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.
Frequently asked questions
Is 27 per cent vertical sky component a planning standard?
No. BRE 209 gives 27 per cent as the point below which a window may begin to look poorly lit in a low-density setting. It is guidance, not a threshold in policy, and in dense urban areas many existing windows already fall below it. Councils and inspectors usually give more weight to the retained ratio against the existing value than to the absolute figure.
What does a retained ratio of 0.8 mean?
It means the proposed value is 80 per cent of the existing value. BRE 209 treats a fall below 0.8 as a loss that occupants are likely to notice, for vertical sky component, daylight distribution and sunlight alike. It is a comparison, so it depends entirely on how the existing baseline was modelled.
Why has average daylight factor disappeared from newer reports?
The 2022 edition of BRE 209 moved interior daylight assessment onto target illuminance, following BS EN 17037. Reports written to the older guidance used average daylight factor instead. If you are comparing an old report with a new one for the same site, you are comparing different measures and the numbers are not interchangeable.
Which rooms have to be assessed?
Habitable rooms: living rooms, dining rooms, kitchens and bedrooms. Bathrooms, stores and circulation space are normally excluded. Check the room uses in the report against the existing floor plans, because assessing a living room as a bedroom lowers the target and improves the reported result.
Do the sunlight criteria apply to every window?
Only to windows within 90 degrees of due south. North-facing windows are not tested for annual probable sunlight hours. If a report shows very few sunlight rows, confirm that the omitted windows fall outside that arc rather than being left out of the sample.
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.