If a daylight and sunlight report has just landed in your inbox, the first read can feel like decoding a foreign language. Between the acronyms, the percentages and the tables of before-and-after figures, it is easy to lose sight of the one question that actually matters: does the scheme pass, and if not, by how much? This glossary walks you through a report section by section so you can read it with confidence.
Daylight and sunlight reports are a routine part of the UK planning system. They are prepared against the Building Research Establishment's guidance, BRE BR 209 (2022), and the British Standard BS EN 17037, and they are used both to test the rooms inside a new development and to check the impact on neighbouring homes. Understanding the vocabulary puts you back in control of your own application - or your objection.
Why the report exists in the first place
Local planning authorities treat access to natural light as a material consideration. When you build an extension, convert a loft or put up a new block of flats, the council needs reassurance that you are not unreasonably darkening a neighbour's windows, and that the new rooms you are creating will themselves be pleasant to live in. A daylight and sunlight report provides that evidence in numbers rather than opinion. If you are new to the subject, our explainer on what a daylight report is and when you need one is a good starting point.
Broadly, every report is doing one or both of two jobs: an external assessment (the effect of your building on other people's light) and an internal assessment (the quality of daylight inside the new rooms). The metrics differ between the two, which is why one report can contain several different tables.
The core acronyms, decoded
Here are the terms you will meet most often. We cover the calculations in more depth in our guide to VSC, NSL and APSH, but this is the short version.
VSC - Vertical Sky Component
VSC measures how much sky a window can "see", expressed as a percentage. Picture standing at the centre of a window and looking out: the more open sky in view and the less obstruction from your own new wall, the higher the figure. The BRE benchmark is a VSC of around 27% for a window with an unobstructed outlook. As a rule of thumb, if a neighbour's window keeps at least 0.8 times its former VSC value after your scheme is built, the loss is considered acceptable. That 0.8 threshold is so central that we gave it a whole article - the 0.8 times rule explained.
NSL - No Sky Line (daylight distribution)
The No Sky Line divides the part of a room that can still see the sky from the part that cannot. It is really a measure of how deep daylight penetrates into a room. In a report you will usually see the percentage of the working plane that retains a view of the sky before and after development. As with VSC, a retained value of at least 0.8 times the previous figure is the usual test.
APSH - Annual Probable Sunlight Hours
APSH applies to sunlight rather than diffuse daylight, and it only matters for windows facing within 90 degrees of due south. It estimates the proportion of the sun's annual track that reaches a window, allowing for typical cloud cover. The BRE targets are 25% of annual probable sunlight hours in total, of which at least 5% should fall in winter (21 September to 21 March). A window that keeps 0.8 times its former APSH, and does not drop below these totals, is generally fine.
Target illuminance and MDF
For the rooms inside a new home, the 2022 guidance leans on BS EN 17037, which sets target illuminance levels - a minimum lux value that a defined fraction of the room should achieve for a set proportion of daylight hours. This has largely replaced the older Average Daylight Factor approach. The change is one of the most significant in the current guidance, and we cover it in what changed in BRE BR 209 (2022) and in our dedicated piece on BS EN 17037.
Reading the tables
Most of a report's substance sits in its tables. Once you know the headings, they become far less intimidating.
- Existing vs Proposed columns. External assessments almost always show a "before" value and an "after" value for each affected window, plus the ratio between them. The ratio is where the pass or fail lives - look for values at or above 0.80.
- Room references. Windows are grouped by the room they serve, often with a use noted (bedroom, living room, kitchen). This matters because kitchens are given less weight than habitable rooms, and purely functional spaces such as bathrooms and hallways are usually excluded altogether.
- Retained percentages. A window showing, say, 0.92 has kept 92% of its former value - a comfortable pass. A figure of 0.71 is a fail against the guideline and will need explaining.
- Absolute values. Do not read the ratio in isolation. A large percentage loss from an already very high starting figure can still leave a perfectly acceptable absolute result, which the BRE guidance explicitly allows an assessor to take into account.
Green, amber, red: what a "fail" really means
A common misunderstanding is that a single transgression sinks an application. In practice the BRE guidance is advisory, not a set of legal limits, and it repeatedly says its figures should be applied flexibly. A handful of minor exceedances - especially to kitchens, secondary windows, or windows that were unusually well-lit to begin with - are frequently accepted by planning officers where the overall picture is reasonable and the context is dense or urban.
What a report should do, when results fall short, is explain why: perhaps the neighbouring window serves a non-habitable room, perhaps the town-centre setting justifies a lower target, or perhaps mitigation such as a redesigned roof form has been introduced. A report that simply lists red figures without narrative is doing only half the job.
Overshadowing and outdoor space
Many reports also assess gardens and communal amenity areas. The BRE test here is that at least half of an amenity space should receive at least two hours of sunlight on 21 March, the spring equinox. If your report includes a shadow plot or a "sun on ground" diagram, this is the standard it is measuring against. Outdoor amenity light is increasingly scrutinised for higher-density housing, so it is worth checking your scheme's gardens as well as its windows.
Daylight report or right to light?
One point of confusion worth clearing up: a planning daylight report is not the same as a right to light assessment. The first informs a council's planning decision against BRE guidance; the second is a matter of private property law, measured in a completely different way, and can lead to injunctions or compensation. If a neighbour has mentioned "rights", read our comparison of right to light versus a daylight report before you respond.
A quick reading checklist
- Identify whether each section is an external (neighbour impact) or internal (new rooms) assessment.
- For external results, scan the retained-value ratios and flag anything below 0.80.
- Check whether flagged windows serve habitable rooms or lesser spaces such as kitchens and bathrooms.
- Read the commentary explaining any exceedances - context and absolute values matter as much as the ratio.
- Confirm that sunlight (APSH) and any amenity overshadowing tests are addressed, not just daylight.
How Fortress Associates can help
If you would rather have an expert read the numbers with you - or prepare a report that anticipates the council's questions before they are asked - we can help. Fortress Associates prepares BRE 2022 and BS EN 17037 daylight and sunlight assessments for homeowners, architects and developers across the UK, with a typical 4 to 5 day turnaround and no advance payment required. Learn more about our daylight report service, see the full list of what we offer, or get in touch with your drawings for a straightforward, jargon-free assessment. Curious about budget first? Our guide to daylight report costs in the UK sets out what to expect.
Sources & further reading
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