Annual probable sunlight hours: the BRE 209 sunlight tests

Annual probable sunlight hours, or APSH, is the measure BRE 209 uses to test whether a window will still get enough direct sun once a neighbouring scheme is built. The headline numbers are 25 per cent of APSH across the year, of which at least 5 per cent must fall in the winter period between 21 September and 21 March. Miss both and the guide treats the loss as noticeable, subject to a further check on how far the figure has dropped. Everything else in a sunlight chapter, the tables, the room schedules, the coloured plans, is presentation wrapped around those two percentages.

Getting the framing right matters more than the arithmetic, because the arithmetic is done by software.

What annual probable sunlight hours actually counts

APSH is the proportion of the sunlight hours that a location could theoretically receive in a year, adjusted for the probability of cloud, that a given window can actually see. It is expressed as a percentage of the total available at that latitude, not as a count of hours, which is why an APSH figure of 28 per cent means something quite different from "28 hours of sun".

The test applies only to windows facing within 90 degrees of due south. A north-facing window is outside the scope entirely, and a report that produces APSH figures for one has either mis-set the orientation or is padding the schedule.

Sunlight is assessed at the window, using the same reference point as the daylight tests: the centre of the window on the inside face of the wall. Obstructions counted include the proposed massing, existing buildings that will remain, and permanent structures. Trees are usually excluded because canopy cover changes, though a mature evergreen belt is worth a sentence of commentary rather than silence.

The winter component and why reviewers look at it first

The 5 per cent winter figure is the part that fails. Between late September and late March the sun sits low, so a modest increase in the height of an opposite building removes a disproportionate share of the winter track while leaving the annual total looking healthy.

A window can comfortably clear 25 per cent APSH annually and still drop to 3 per cent in winter. That is a fail on the guideline, and it is also the condition occupants complain about, because winter is when a living room with no sun feels cold and grey.

So when reading a sunlight table, look at the winter column before the annual one. If the report only gives annual figures, the assessment is incomplete and should go back.

The retained-value test

Meeting the absolute targets is not the whole test. BRE 209 adds a proportional check: where a window falls below the target, the guide asks whether the retained value is less than 0.8 times the former value, in either the annual or the winter period, and whether the annual reduction exceeds four percentage points of APSH.

The practical effect is that a window already at 12 per cent APSH, in a dense urban street, is not automatically a problem if the scheme takes it to 11. A window dropping from 40 per cent to 20 per cent is, even though 20 is not far off the target, because it has lost half its sun.

Reviewers should watch for reports that quote only one of these limbs. Quoting the 0.8 factor without the absolute percentages, or the percentages without the ratio, produces a conclusion the guide does not support.

Which rooms the test applies to

BRE 209 directs the sunlight test at main living rooms of dwellings, and at conservatories. Bedrooms are treated as less important, and kitchens sit somewhere between, with the guide warning against blocking too much sun from either rather than setting them a target.

That hierarchy is where most disputes start. Where a flat has a combined living, kitchen and dining space served by one window, the room is a living room and the window is tested. Where a developer has labelled a plainly habitable front room "study" on the submitted plans, expect an objector's consultant to test it anyway and expect the case officer to ask why the labels changed between the planning drawings and the assessment.

Non-residential rooms are outside the guidance unless the local plan says otherwise. Schools, care homes and healthcare buildings often attract local policy that reaches further, and a daylight and sunlight assessment should state which policy it is answering as well as which BRE test it applies.

Sunlight to gardens and amenity space

Gardens and communal amenity areas get their own check, and it is a different metric altogether. The guide looks at 21 March and asks whether at least half of the area receives at least two hours of direct sun. Where it does not, the second limb asks whether the area able to receive those two hours has fallen below 0.8 times its former extent.

Two hours on the equinox is a low bar in an open suburban garden and a hard one in a courtyard between four-storey blocks. It is also the test that decides whether a scheme needs a separate overshadowing study rather than a paragraph inside the main report.

Illustrative example: a rear courtyard of 200 m2 behind a terrace, where 140 m2 currently receives two hours on 21 March. A proposed three-storey rear block reduces that to 96 m2. The area is now below half the courtyard and below 0.8 times the former figure, so both limbs are engaged and the assessment has to explain the residual amenity, not simply report the number.

What the 2022 edition changed

The third edition of BRE 209, published in 2022, changed the presentation more than the sunlight arithmetic. Daylight assessment was aligned with BS EN 17037, moving the emphasis from average daylight factor towards illuminance-based targets and spatial daylight measures. The sunlight tests, APSH and the two-hour equinox check on amenity space, carried across largely intact.

The other change was tonal, and it gets quoted selectively. The guide is explicit that its numerical values are advisory, that they were derived largely from low-rise suburban settings, and that they should be applied flexibly with regard to context. Applicants read that as permission to fail the targets. Objectors read it as meaningless. Both are wrong. Flexibility means the decision maker weighs the shortfall against the context, the townscape and the policy, which is a judgement that has to be argued in the planning balance rather than asserted in a table.

Because the guide is a copyrighted BRE publication, an assessment should cite the edition and section rather than reproduce the text, and any reviewer signing off a draft should be working from the current edition.

Reading an APSH table without being misled

Six habits catch most of the problems:

1. Check the baseline. An assessment run against a cleared site rather than the existing building understates loss. The comparison should be existing condition against proposed.
2. Check the window schedule against the floor plans, room by room, and query any room whose use has been relabelled.
3. Look for the winter column, and treat its absence as a defect.
4. Check whether both limbs of the retained-value test are reported for every affected window.
5. Count how many windows fail rather than reading the percentage of windows that pass, since a scheme with 400 compliant windows and 11 failures on one flank is a scheme with 11 arguments.
6. Confirm the modelled massing matches the current drawings, including plant, parapets and lift overruns.

Daylight and sunlight work is a specialism, and councils expect a suitably qualified author. PlanScope drafts the structure and the policy framing of a daylight and sunlight report, and flags where site-specific modelling output is missing rather than inventing it, but the modelling itself and the professional judgement on residual harm belong to the consultant who runs it. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

If you are reviewing or signing off drafts for clients, the professional review route is where PlanScope sends reports for sign-off. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.