Annual probable sunlight hours and the winter component
The annual probable sunlight hours test has a separate winter component, and a window only fails where the annual, winter and proportional tests all break together.
Annual probable sunlight hours measures how much of the sun's possible annual track actually reaches a point on a window, expressed as a percentage. The BRE guidance sets two figures rather than one: a whole-year value and a separate winter value covering 21 September to 21 March. A window is only judged to be adversely affected where it fails the annual figure, the winter figure and the proportional test together.
That conjunctive structure is the part consultants get wrong when reading a rival report, and it is the part that decides most sunlight arguments at committee.
What the measure counts
The sunlight availability indicator plots the sun's position across the year onto a diagram divided into spots, each spot representing a fixed share of the total probable sunlight hours at that latitude. Probable, because the calculation already discounts for average cloud cover rather than assuming clear skies.
An assessor places the indicator at the centre of the window on the outside face, plots the obstructions, counts the spots that remain visible, and reports the result as a percentage of annual probable sunlight hours. The winter figure counts only the spots falling within the winter half of the year.
Two things follow from the method. The figure relates to a point on the window rather than to conditions inside the room, so it says nothing directly about how sunny the living room feels. And it is sensitive to the accuracy of the obstruction model, which is why the baseline matters so much and why a model built from an out-of-date survey produces numbers nobody can defend.
The annual and winter thresholds
The guidance in BRE 209, now in its 2022 edition, treats a window as likely to suffer a noticeable loss of sunlight where all of the following apply:
- It receives less than 25 per cent of annual probable sunlight hours.
- It receives less than 5 per cent of annual probable sunlight hours in the winter months, from 21 September to 21 March.
- It receives less than 0.8 times its former value in either the annual or the winter period.
- The reduction over the whole year is more than 4 per cent of annual probable sunlight hours.
All four. A window that drops to 22 per cent annual but holds 6 per cent in winter has not failed the test as drafted, and a window that falls from 40 per cent to 30 per cent has not failed either, because it stays above 25 per cent.
The winter component exists because winter sun is the sun people notice losing. A low December sun comes in at a shallow angle, reaches deep into a room, and is exactly the light that a new building opposite removes first. A scheme can preserve a respectable annual figure while stripping out the winter component almost entirely, and the separate 5 per cent threshold is there to catch that.
Where an assessment reports only the annual figure, ask for the winter one. It is the more revealing number on a north-south street.
Which windows are tested
BRE applies the sunlight test to main windows serving living rooms in existing dwellings. Kitchens and bedrooms are treated as less important, on the reasoning that sunlight matters most where people sit during the day, and the guidance says so explicitly.
That gives assessors a judgement to make about room use, and it is a judgement that moves results. A room recorded as a bedroom is excluded from the sunlight test altogether, and the same room recorded as a living room may generate a breach. The note on room use assumptions covers how those decisions are made and evidenced, and on any contested scheme the assumptions should be set out property by property rather than summarised.
Windows serving proposed dwellings within the scheme itself are assessed on the same basis, though authorities vary in how strictly they apply the figures to new build. Amenity space is a separate test entirely, measured as sun on the ground on 21 March, which is what an Overshadowing Study addresses.
Orientation and when the test falls away
The sunlight test only applies to windows facing within 90 degrees of due south. A window facing north-east, north or north-west receives no direct sun for most of the year at British latitudes, so there is nothing for a new building to take away.
This is not a loophole, and objectors sometimes read it as one. It also produces the result that a terrace of houses on the north side of an east-west street is largely unaffected by a tall building opposite in sunlight terms, while the terrace on the south side may be badly affected. Set the orientation out clearly on a plan in the report, because a committee reading a table of numbers with no compass cannot tell why half the windows were excluded.
Where a window faces close to the 90 degree limit, say 85 degrees from south, the calculated figures will be low in the baseline as well as the proposed case, and the proportional test does most of the work.
The proportional test and its limits
The 0.8 ratio and the 4 per cent reduction exist to stop the absolute thresholds catching windows that were always poorly sunlit. A basement window in a dense terrace may receive 8 per cent annual sunlight before anything is built. Applying a 25 per cent threshold to it would condemn every scheme in the street.
The limit of that reasoning is that it rewards existing poor conditions. In parts of central London, baseline values are so low that almost any new building passes the proportional test while the residents' actual experience gets worse. BRE deals with this by stating that its numerical values are advisory and should be applied flexibly, with the context of the site taken into account, and the London Plan takes the same approach for dense urban locations.
That flexibility cuts both ways, and it is where the real argument happens. An applicant will say the thresholds were written for suburban layouts and should be relaxed in a town centre. An objector will say flexibility cannot mean abandoning the test. Neither position is unreasonable, and the inspector usually looks at how many windows fail, by how much, and whether the affected rooms are the main living space of a home.
Reading the results table
A usable table has one row per window, and it shows: the property address, the window reference keyed to a plan, the room use assumed, the orientation, the existing annual and winter values, the proposed annual and winter values, the ratio for each period, and the absolute reduction. Anything less and the reader cannot check the conclusion.
Watch for four things in someone else's table. Windows reported as compliant where only the annual figure is given. A room use schedule with no source. Aggregate pass rates quoted as a percentage of all windows, which hides a cluster of severe failures in one property. And a baseline that includes an existing building on the application site modelled at its full height when it is derelict, or excluded when it is standing.
Presentation matters as much as the arithmetic once the case reaches members, and the guidance on presenting daylight results a committee can read sets out how to turn the table into something a non-specialist can use.
Design responses that recover sunlight
The moves that work are geometric, and they have to happen early:
- setting back upper floors on the southern edge of the site, which recovers winter sun disproportionately because low-angle light is blocked by height close to the boundary
- chamfering or stepping a corner where a block sits due south of a run of living room windows
- widening a gap between blocks so that the winter sun track passes through it, rather than distributing the same width across two narrow slots
- reorienting the proposed building slightly to open the south-east or south-west approach angle
- relocating the affected room use within your own scheme, which works for proposed dwellings and does nothing for existing ones
Rooflights, light shelves and internal alterations do not help, because the test measures light at the window rather than in the room. And a taller, thinner block is often worse than a lower, wider one for winter sunlight, which is the opposite of the intuition most massing exercises start from. Testing this at the massing stage, using the approach described in the note on obstruction angles, costs far less than reworking a consented scheme.
Where the numbers are marginal, commission the full Daylight and Sunlight Assessment before the design is fixed rather than as validation evidence afterwards.
This is general guidance, not legal advice. Daylight and sunlight modelling should be carried out by a suitably qualified specialist, and BRE 209 is advisory guidance to be applied with judgement. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What are the APSH thresholds in BRE 209?
A window is treated as likely to suffer a noticeable loss where it receives less than 25 per cent of annual probable sunlight hours, less than 5 per cent in the winter months from 21 September to 21 March, less than 0.8 times its former value in either period, and has an annual reduction greater than 4 per cent. All four conditions have to apply together.
Why is there a separate winter sunlight figure?
Low winter sun enters at a shallow angle and reaches deep into a room, so it is the light people notice losing. A new building opposite removes that first. A scheme can hold a reasonable annual figure while stripping out almost all winter sun, and the separate 5 per cent threshold exists to catch that outcome.
Which windows are assessed for sunlight?
Main windows serving living rooms in existing dwellings, where the window faces within 90 degrees of due south. Kitchens and bedrooms are treated as less important in the BRE guidance, and windows facing north-east through to north-west are excluded because they receive little direct sun at British latitudes in any event.
Can a scheme fail the annual test but still be acceptable?
Yes. BRE states that its numerical values are advisory and should be applied flexibly with the context of the site in mind, and policy in dense urban areas such as the London Plan takes the same approach. Decision makers usually look at how many windows fail, by how much, and whether the rooms are main living spaces.
Do rooflights or internal changes improve an APSH result?
No. The test measures sunlight reaching a point on the outside face of the window, so internal alterations, light shelves and rooflights change nothing in the calculation. The effective responses are geometric: setting back upper floors on the southern edge, chamfering corners due south of living room windows, and widening gaps between blocks.
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.