BRE BR 209 (2022) explained

VSC, NSL and APSH: what the numbers mean

Every daylight and sunlight report for a planning application comes down to three sets of numbers. Each one answers a plain question about a neighbour's window.

VSCVertical Sky Component

How much sky the window can still see.

27% or 0.8× former

NSLNo Sky Line

How much of the room behind it still gets daylight.

0.8× former area

APSHAnnual Probable Sunlight Hours

How much sunshine that window still receives.

25% year / 5% winter

The tests come from the Building Research Establishment's guide Site layout planning for daylight and sunlight: a guide to good practice, BR 209, now in its 2022 edition. Below is what each number measures, the thresholds councils look for, and what happens when a proposal falls short.

VSC: how much sky the window can see

Vertical Sky Component measures the sky visible from the centre of a window, as a percentage of the light from an unobstructed sky. A window facing open countryside reaches about 39%. Put a building opposite and the figure falls.

The threshold

27% or more is generally enough. Below that and below 0.8× its former value, the loss is likely to be noticeable.

Both halves matter, and this is where most misunderstandings start. A window already at 12% in a dense terrace will never reach 27%, so the question becomes the relative one: does your extension take it below 9.6%? A window dropping from 38% to 30% has lost a lot of sky, but still sits above 27%, so it passes.

VSC is measured at the outside face of the window, so it describes the obstruction, not the room. A window with a healthy VSC can still serve a gloomy room — which is why the second test exists.

VSC is measured at the centre of the neighbour's window. The yellow wedge is the sky the proposed extension or development takes away.

Top: a slice through the neighbour's house. Bottom: the same room from above. The no-sky line is measured at table-top height, 0.85 m above the floor, and marks how far into the room the sky can still be seen.

NSL: how much of the room still gets daylight

The No Sky Line divides the part of a room that can see the sky directly from the part that cannot. Stand in the back corner of a deep kitchen, look at the window, and if you cannot see any sky from there, you are behind the no-sky line.

The threshold

If the area receiving direct skylight falls below 0.8× its former size — a loss of more than 20% — occupants are likely to notice.

NSL catches proposals that VSC lets through. It needs the internal layout, so it is assessed where floor plans of the neighbouring property are available or can be reasonably inferred, and applied to the rooms that matter: living rooms, dining rooms and kitchens.

BR 209 (2022) also notes that where a kitchen worktop sits against a wall, daylight to that working surface is judged through the no-sky line rather than as a test of its own.

APSH: how much sunshine the window still gets

Annual Probable Sunlight Hours measures direct sunshine rather than diffuse daylight, so it applies only to windows facing within 90° of due south. A north-facing window never receives direct sun in the UK, so there is nothing to assess.

The threshold

At least 25% of annual probable sunlight hours, including at least 5% between 21 September and 21 March. A loss counts as noticeable only if the window falls below those figures and below 0.8× its former sunlight and the annual reduction is more than 4%.

Because there is both an annual and a winter figure, a scheme can pass comfortably on summer sun and still fail in winter — common where an extension sits on the south side of a neighbour, blocking a low winter sun while casting no meaningful shadow in June.

BRE sunlight guidelines

25%or more of annual probable sunlight hours

5%or more of annual probable sunlight hours falling between 21 September and 21 March

The test applies mainly to main living-room windows facing within 90° of due south.

For existing neighbours, the BRE also looks at the change. A loss is likely to be noticed where sunlight falls below these figures, falls below 0.8 times its former value, and the annual loss is more than 4% of annual probable sunlight hours.

Sun heights at solar noon for London, shown for illustration. The BRE test counts probable sunlight hours across the whole year, and separately for the winter half between 21 September and 21 March.

The tests side by side

VSC, NSL and APSH are the three tests for neighbouring windows. Sun on Ground covers gardens and outdoor space, and two further tests apply inside new homes.

Test	Measures	Applies to	Guidance benchmark
VSC	Sky visible at the window	Neighbours: all main windows	27%, or 0.8× former value
NSL	Room area receiving direct skylight	Neighbours: rooms where the layout is known	0.8× former area (20% loss)
APSH	Direct sunshine across the year	Neighbours: windows within 90° of due south	25% annual and 5% winter; 0.8× former; 4% annual loss
SOG	Sun on Ground: sunlight reaching gardens and amenity space	Neighbours’ gardens, and the scheme’s own outdoor space	At least half the area receives 2 hours of sun on 21 March; for existing gardens, 0.8× former area
SDA	Spatial Daylight Autonomy: daylight inside a room	Internal: rooms in new homes	100 lux in bedrooms, 150 lux in living rooms, 200 lux in kitchens, across at least half the room for at least half of daylight hours
SE	Sunlight Exposure: direct sun inside a home	Internal: new homes	At least 1.5 hours of sunlight on 21 March to a habitable room, preferably the main living room

Sun on Ground is used two ways: to check the effect on a neighbour’s garden, and to show how well a scheme’s own gardens and amenity spaces perform. The same test applies to trees and their shade.

What if a proposal fails a test?

A number below the benchmark is not an automatic refusal. BR 209 is guidance, not regulation, and says its figures should be applied flexibly according to context — dense urban areas are explicitly recognised as places where the targets may not be achievable, and several London boroughs say so in their own policies.

What a council needs is evidence: how far below, how many rooms, whether they are habitable, and whether the design could reasonably reduce the impact. In practice small changes fix most problems — a few hundred millimetres off the depth, a flank wall set back from the boundary, a lower eaves, a repositioned rooflight. Finding out before submission costs far less than a refusal and a resubmission.

Common questions

Is a low VSC on its own a reason for refusal?

No. Officers weigh the extent of the loss, the number of rooms affected and the character of the area alongside everything else in the application.

Do these tests apply to the new rooms I'm building?

VSC, NSL and APSH protect existing neighbouring properties. Light inside new homes is assessed with SDA and SE under BS EN 17037, which is a separate exercise.

Do you need to visit the property?

No. The assessment is built from your drawings together with published mapping and survey data, which is why the fee is fixed and the turnaround is short.

How much does an assessment cost?

From £499 + VAT for a small terraced house with a single-storey rear extension affecting one neighbour; most reports are £675–£935 + VAT. The price calculator gives your exact fixed fee.

How long does a report take?

Usually 4–5 working days from receiving your drawings.

Get the numbers for your project

BRE-compliant assessments for planning. Fixed fee agreed before any work starts, usually delivered in 4–5 working days.

Get your fixed fee 01962 809155

More background: our plain-English guide to the BRE guidelines covers the 25° and 45° tests, and the assessment page explains what a full report includes.