BRE Daylight & Sunlight Guidance Explained: VSC, APSH & NSL
If a council has asked for a daylight and sunlight assessment, or a scheme next door has you worried about your light, you’ll quickly run into a wall of initials: VSC, NSL, APSH, ADF, BRE, BS EN 17037.
This guide translates all of them into plain English — what each one measures, the target it has to hit, and what actually changed when the guidance was rewritten in 2022.
Key takeaways
- A daylight & sunlight assessment is a planning document, judged against the BRE guide BR 209 (3rd edition, 2022) and the standard BS EN 17037.
- It answers two questions: how much light the scheme takes from neighbours, and how much light the new homes themselves receive.
- Light lost by neighbours is tested with VSC (27%), the no-sky line, APSH (25% + 5% winter) and overshadowing (2 hours on 21 March).
- Light into new homes is now judged on illuminance / daylight-factor targets — the old average daylight factor (ADF) was dropped in 2022.
- It is not the same as right to light, a separate legal right enforced through the courts.
What is a daylight & sunlight assessment?
A daylight and sunlight assessment is a technical report, prepared for a planning application, that measures how much natural light a proposed building will take from its neighbours and how much light the new homes inside it will get.
It’s how a local planning authority judges the light impact of a scheme objectively rather than on gut feel. Almost all of it rests on one document: the Building Research Establishment’s BR 209, ‘Site layout planning for daylight and sunlight: a guide to good practice’.
The current third edition was published in June 2022 to bring the guide into line with the European daylight standard BS EN 17037. Between them, they set the methods and the numbers everyone — councils, architects, developers and surveyors — is expected to use.
When you need one depends on the scheme and the borough. Many authorities require an assessment once a development crosses a size threshold or could affect neighbours — Camden, for example, asks for one where there are 10 or more new residential units.
The only reliable answer is your council’s local validation list, so check that (or ask a surveyor) before you design too far.
Do you actually need one?
You’re most likely to need a daylight & sunlight assessment if your scheme is a new build, an upward extension or a higher-density plot that could overshadow neighbours — or if it contains new homes whose own light must be demonstrated. Commissioning it early matters: the findings can steer massing and building heights before the design is fixed, rather than forcing an expensive redesign later.
The two halves of every assessment
Almost every misunderstanding about daylight and sunlight comes from mixing up its two separate jobs. A good report keeps them apart:
Light lost by neighbours
How much daylight and sunlight your proposal removes from the buildings and gardens around it. Tested with:
- VSC — daylight to a neighbour’s window
- No-sky line — daylight spread inside their rooms
- APSH — sunlight to their living rooms
- Overshadowing — sun to their gardens
Light into new homes
Whether the homes you’re creating will be bright enough to live in. Since 2022 this is tested with:
- Target illuminance / spatial daylight autonomy, or
- a median daylight factor target
- plus direct sunlight on 21 March to living spaces
The metrics, explained
These are the measures that decide whether a scheme passes. Open each one below, then see them side by side in the table.
The headline daylight test for neighbours — how much sky a window can ‘see’.
Measures: the daylight reaching the outside face of a neighbour’s window, as a percentage of visible sky.
Target: 27% or more is generally good daylight.
Pass rule: if a scheme drops the VSC below 27%, it should still keep at least 0.8× (80%) of the former value. As a rule of thumb, 27% roughly corresponds to a 25° obstruction angle above the window.
Not how much light reaches the window, but how far it spreads into the room.
Measures: the line that separates the part of a room that can see the sky from the part that can’t — i.e. how deep daylight reaches.
Target: the daylit area shouldn’t shrink materially because of a new development.
Pass rule: if reduced, the lit area should stay at 0.8× (80%) or more of its former size. A big loss here means gloomy room backs even if the window still tests well.
The sunlight test — direct sun, not diffuse sky light.
Measures: the proportion of direct sunlight a window could receive over a year. Only applies to windows within 90° of due south.
Target: 25% of annual probable sunlight hours (about 1,486 hours), including at least 5% in winter.
Pass rule: if reduced below those figures, the window should keep 0.8× (80%) of its former sunlight. Used mainly for existing living rooms and conservatories.
Sunlight on the ground rather than at a window.
Measures: how much of a garden or amenity area still gets sun once the new building is in place, tested on the spring equinox (21 March).
Target: at least half (50%) of the amenity area should still receive at least 2 hours of sunlight on 21 March.
Pass rule: if it falls below that, the sunlit area should remain at 0.8× of its previous size.
| Metric | What it measures | BRE target | If reduced |
|---|---|---|---|
| VSC | Daylight to a neighbour’s window | ≥ 27% | Keep ≥ 0.8× former value |
| No-sky line | Daylight spread within a room | Area not materially reduced | Keep ≥ 0.8× former area |
| APSH | Direct sunlight to a window (within 90° of south) | ≥ 25% annual, incl. ≥ 5% winter | Keep ≥ 0.8× former value |
| Overshadowing | Sunlight to gardens / amenity | ≥ 2 hrs on 21 March over ≥ 50% of area | Keep ≥ 0.8× former area |
These are guidelines, not hard limits. BR 209 is explicit that the targets can be applied flexibly to suit the development and its setting — a dense city-centre plot is judged differently from a suburban street. That interpretation is exactly where an experienced surveyor earns their fee.
What changed in 2022 (ADF is gone)
The biggest change in the 2022 guidance is how light into new homes is judged. The old average daylight factor (ADF) was retired and replaced by illuminance-based methods from BS EN 17037.
For years, daylight to a new dwelling was measured with a single figure — the average daylight factor, which could be worked out with a formula. The 2022 edition drops it. New homes are now assessed with one of two methods:
Target illuminance
A required light level that must be met over a proportion of the room for at least half of daylight hours — known as spatial daylight autonomy. It uses climate-based modelling with real weather data for the site, so results vary by location and orientation.
Median daylight factor
A daylight-factor target calculated under a standard overcast sky. It doesn’t change with orientation, though the target shifts by latitude. It replaces the old mean (ADF) figure with a median one.
Either method is acceptable, and both need specialist software running calculations across a grid of points in each room. Two practical consequences: modern reports are more detailed than older ones, and a scheme assessed under the new rules can’t simply be compared with one assessed under the old ADF.
Sunlight into new homes changed too — it’s now checked by the amount of direct sun a window gets on 21 March, rather than annual probable sunlight hours.
Daylight & sunlight vs right to light
They’re two completely different things. A daylight & sunlight assessment is a planning matter judged against BRE good practice. Right to light is a private legal right, enforced through the courts.
This is the single most expensive misunderstanding in development. A scheme can pass every BRE daylight and sunlight test and still infringe a neighbour’s right to light — or satisfy planning yet face an injunction or a six-figure damages claim. Planning permission doesn’t extinguish the private right.
The two must be run in parallel, which is why we advise on both. If a nearby scheme threatens your light, or you’re a developer weighing the risk, see our guide to right to light and our right to light service.
Solar panels & overshadowing
The 2022 guide added something older editions didn’t cover: solar panels. As rooftop installations have multiplied, BRE now gives advice on the best positioning of panels and, importantly, numerical targets for the loss of solar radiation where a new development would shade an existing panel.
If your scheme could overshadow a neighbour’s array — or theirs could shade yours — it can now be assessed in the same report, alongside the usual garden-overshadowing test on 21 March.
A quick myth: the ‘45-degree rule’
You’ll often hear that light is judged by a ‘45-degree rule’. That’s a rough planning rule of thumb some officers use for a first look at an extension — it’s not the BRE technical test and has nothing to do with VSC, APSH or the legal right to light. Don’t rely on it to tell you whether a scheme will pass a proper assessment.
Frequently asked questions
Daylight is diffuse light from the whole sky, available all day and measured with VSC and the no-sky line. Sunlight is direct light from the sun’s position, measured with APSH and the 21 March overshadowing test, and it only matters for windows and spaces facing roughly south. An assessment covers both.
A 3D model of the scheme and its surroundings, before-and-after results for each affected window and amenity space against the BRE metrics (VSC, NSL, APSH, overshadowing), an assessment of the new homes’ own daylight and sunlight, and a plain-English conclusion on where the scheme meets guidance and where it needs attention.
Often, yes — especially for new builds, upward extensions and higher-density schemes, or where new homes’ light must be demonstrated. Requirements vary by borough, so check your council’s local validation list. Commissioning one early lets the findings shape the design before problems are baked in.
VSC is the proportion of visible sky measured at the centre of the outside face of a window, expressed as a percentage. A value of 27% or above is generally good; if a development reduces it, the window should keep at least 0.8× (80%) of its former VSC. It’s calculated from a 3D model rather than measured on site.
It can. Since the 2022 guidance, BRE gives advice on solar-panel positioning and targets for the loss of solar radiation where a new building would overshadow an existing panel. If your scheme could shade a neighbour’s array, that’s assessed alongside the daylight, sunlight and garden-overshadowing tests.
No. A daylight & sunlight assessment is a planning document judged against BRE good practice. Right to light is a separate private legal right enforced through the courts, using a different method (the Waldram analysis). A scheme can pass one and still fall foul of the other.
It depends on the size and complexity of the scheme, the number of surrounding properties to model, and whether you need early design advice as well as a formal report. Every project is quoted individually — send us the details for a tailored, no-obligation quote.
Planning a scheme? Get the light right early.
Blackacre’s RICS-regulated surveyors prepare BRE daylight, sunlight and overshadowing assessments across London, Surrey and Sussex — and steer your design so light doesn’t become a planning problem.
Sources: BRE, Site layout planning for daylight and sunlight: a guide to good practice (BR 209, 3rd ed, 2022); BS EN 17037:2018 Daylight in buildings; RICS Built Environment Journal, ‘BRE revises guidance on access to daylight and sunlight’ (2024). General guidance, not scheme-specific advice — always check your local planning authority’s validation requirements.