Annual probable sunlight hours: a worked example

Annual probable sunlight hours is the sunlight test in BRE 209, and it only applies to windows facing within 90 degrees of due south. The annual probable sunlight hours example below works through a single living room window, because the test has three limbs that all have to be met before a loss counts as noticeable, and reports regularly apply one of them and stop.

Sunlight is assessed separately from daylight. A window can pass every daylight test and fail on sunlight, or the reverse.

What the measure describes

Annual probable sunlight hours expresses, as a percentage, how much of the sunlight that could theoretically reach a point actually does once cloud cover and obstructions are taken into account. The total hours available vary with latitude, so the underlying figure comes from the sunlight availability indicator for the location rather than from a national constant. Percentages are always percentages of that local total.

Two periods are used. The whole year, and the winter months from 21 September to 21 March. Winter is separated out because that is when low sun angles make obstruction most noticeable and when sunlight does most for perceived warmth and amenity.

The assessment point is the centre of the window on the inside face of the wall. Orientation decides whether the test applies at all: a window facing more than 90 degrees away from due south is outside the scope, which excludes most north-facing windows from the sunlight analysis entirely.

BRE guidance treats sunlight as most important in main living rooms, and less important in kitchens and bedrooms. That is a judgement about the room, not an exemption, and a report should say which rooms it has assessed and why.

The thresholds, and all three of them

The target condition is that a window receives at least 25 per cent of annual probable sunlight hours over the year, including at least 5 per cent in the winter period.

Where a proposal takes a window below those figures, three further conditions all have to be satisfied before BRE treats the loss as noticeable to the occupants:

1. The window receives less than 25 per cent of annual probable sunlight hours over the year, including less than 5 per cent during the winter months.
2. The window retains less than 0.8 times its former sunlight hours in either period, meaning a proportional loss of more than 20 per cent.
3. The reduction over the whole year is more than 4 per cent of annual probable sunlight hours in absolute terms.

Miss any one of those and the loss does not meet the BRE definition. This is where reports go wrong in both directions: an objector's report that stops at the first limb overstates the harm, and an applicant's report that quietly omits the winter figure understates it.

A worked before and after

The following is illustrative. It uses round numbers to show the method rather than to describe any real site.

A ground-floor living room window in a two-storey terrace faces due south across a rear garden. A three-storey building is proposed on the plot behind.

Run the three limbs. The proposed figures are below 25 per cent for the year and below 5 per cent for winter, so the first limb is met. The window retains 21 divided by 42, which is 0.50 of its former value, well below 0.8, so the second limb is met. The absolute annual reduction of 21 per cent of annual probable sunlight hours is above 4 per cent, so the third is met. All three limbs are satisfied, and the loss would be noticeable.

Now change one number. If the proposed whole-year figure were 36 per cent rather than 21 per cent, with winter at 9 per cent, the first limb fails because the window still exceeds both targets, and the analysis stops there regardless of how large the percentage change looks.

That is the point of the sequence. A big-sounding percentage reduction from a generous starting position often does not engage the test at all.

The proportional limb in practice

The 0.8 factor is the limb that decides most real cases, because urban windows frequently start below the 25 per cent target. A window already receiving 18 per cent fails the target before anything is built, so the question becomes how much of what it has is being taken.

Report the retained proportion explicitly for every window assessed, in both periods. A table that gives only before and after percentages forces the reader to do the arithmetic, and a planning committee will not.

Where a window starts below the target and the proportional loss is modest, say so plainly and explain the existing condition. The honest framing is that the window is already poorly served by sunlight for reasons that predate the application, and the proposal makes a small further difference. That argument succeeds far more often than an attempt to show no effect at all.

Windows the test does not reach

Four exclusions come up repeatedly. Windows facing more than 90 degrees from due south are outside the test. Windows serving rooms where sunlight is not expected, such as a utility or a circulation space, are not normally assessed. Rooflights are handled differently because their obstruction geometry is not the same. And the test applies to existing dwellings and to proposed dwellings, not to commercial windows, where sunlight is generally not a planning expectation.

Amenity space is a separate assessment again. The garden or communal area test looks at whether at least half the area receives at least two hours of sunlight on 21 March, which is the measure that usually sits in an Overshadowing Study rather than in the window analysis.

Reporting alongside the daylight results

Present sunlight and daylight separately and then draw them together. A short conclusion table per room, listing the daylight result, the sunlight result and the retained proportions, lets an officer see the pattern without reading two appendices.

State the modelling assumptions: the survey source for the existing buildings, whether balconies and overhangs are modelled, the reflectance values, and whether the baseline is the existing site or a consented scheme. The choice of baseline changes the answer more than any other input, and the reasoning behind it is set out in the mirror image baseline in a daylight assessment.

A Daylight and Sunlight Assessment carries weight with officers when the numbers are reported in full, including the windows that fail. Selective reporting is spotted, and it damages the credibility of the windows that do pass.

Where the test is misapplied

Three habits to avoid. Treating the 25 per cent figure as a standard to be achieved in all cases, when BRE presents its guidance as advisory and flexible in dense urban settings. Applying the test to north-facing windows and reporting a pass, which tells the reader nothing. And describing a result as compliant without naming which limbs were tested.

Say which edition of BRE 209 the assessment follows, because the guidance was revised and the handling of urban context changed with it. Then explain the result in words a neighbour could understand, alongside the Design and Access Statement narrative about massing.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Daylight and sunlight assessment is a report type where councils usually expect a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.