Daylight modelling dates and the benchmark days

Sunlight assessment in England runs on three fixed dates: 21 March, 21 June and 21 December. Daylight assessment, by contrast, is not tied to a date at all, because skylight is treated either geometrically or through a whole-year weather file. Confusing the two is the reason so many submitted studies contain shadow plots nobody asked for and omit the one plot the officer needs.

Why fixed dates rather than averages

An averaged sun position tells you nothing useful. The sun's altitude at noon in London swings from roughly 15 degrees at the winter solstice to roughly 62 degrees at the summer solstice, and a shadow cast at 15 degrees is four or five times the length of the same obstruction's shadow at 62 degrees. Average those and you describe a condition that never occurs.

So the guidance in BRE 209, Site Layout Planning for Daylight and Sunlight, uses specific dates that can be reproduced by anyone with the same model. Two people running the same geometry for 21 March will produce the same shadow outline. That reproducibility is the whole point, and it is why a report that models "a typical spring day" is unusable.

The equinox as the standard assessment date

21 March is the date that carries the test everyone argues about. BRE 209's criterion for amenity areas is that at least half of an open space should receive at least two hours of sunlight on 21 March.

The equinox is chosen because it sits in the middle of the range and because the geometry is clean: the sun rises close to due east, sets close to due west, and the day is roughly twelve hours long. A space that gets two hours of March sun over half its area will be comfortable in summer and poor in December, and that is a reasonable definition of an adequate amenity space in this climate.

The criterion applies to gardens, courtyards, communal amenity space, children's play areas and, where a council asks, allotments. It is a test of area over time, so the output has to be an area plot with the two-hour contour drawn, and a stated percentage. A list of hours at sample points does not answer it.

Midsummer and midwinter as the range

21 June and 21 December are not pass or fail dates. They are context, and they are what stops an objector and an applicant talking past each other.

The midwinter plot shows the worst case. It is the plot neighbours most want to see, and offering it without being asked is usually to the applicant's benefit, because the expectation of a long shadow in December is already in the room. What damages a case is a study that stops at 21 March and looks as though it is hiding something.

The midsummer plot matters where the concern is the opposite of overshadowing: solar gain on a glazed facade, overheating in a courtyard, or usability of a roof terrace. Where the scheme has an overheating question, the June geometry links the daylight work to the Part O assessment.

How annual and winter sunlight periods are defined

Window sunlight uses a different metric from sun on ground, and it uses periods rather than single days.

Annual probable sunlight hours, APSH, is the proportion of the sunlight hours a window would receive in a year if there were no obstruction, adjusted for the probability of clear sky from long-term meteorological records. The BRE criteria are that a main window to a living room should receive at least 25 per cent of annual probable sunlight hours, of which at least 5 per cent falls in the winter period.

The winter period is defined as 21 September to 21 March. That definition is the part people get wrong, and it is worth writing into the report explicitly, because a reviewer who assumes a calendar winter will not reconcile your figures.

One further limit. The sunlight test applies only to windows facing within 90 degrees of due south. A north-facing window is not assessed for sunlight at all, which saves a great deal of unnecessary modelling on a courtyard block.

Shadow plots and the times of day modelled

Shadow plots are produced at intervals through each assessment day. Hourly intervals from sunrise to sunset on 21 March is the standard expectation, with the same treatment on 21 June and 21 December where those dates are included.

Three practical points decide whether the plots get used or ignored:

1. Plot the existing and proposed conditions on the same sheet at the same scale, so the change is readable without flicking between pages.
2. Label every plot with the date, the clock time and whether it is British Summer Time or Greenwich Mean Time. A 21 June plot at 14:00 GMT is a different picture from one at 14:00 BST, and the ambiguity invites a challenge.
3. Include the two-hour cumulative contour as a separate drawing, because that is the drawing the criterion is actually about.

Existing vegetation raises a modelling question of its own, and our note on tree shading in these studies covers how deciduous canopies are handled.

Presenting results for the right dates

An Overshadowing Study for a residential scheme should contain, at minimum, the 21 March hourly series, the 21 March two-hour contour with the percentage of each amenity area meeting the criterion, and a table listing each space with its area and its result.

Where windows are also affected, the Daylight and Sunlight Assessment carries the APSH figures with the annual and winter percentages side by side for each assessed window, plus the retained value as a proportion of the existing value where the guidance's alternative test applies.

Keep the two documents consistent on the model. Same geometry, same terrain, same treatment of surrounding buildings, same date conventions. A discrepancy between the sun on ground model and the window model is the single most common reason an officer asks for a resubmission rather than a clarification.

Where councils ask for additional dates

Some authorities and some sites bring extra dates into play, and the request is usually reasonable.

Where an amenity space is used seasonally, a council may ask for 21 April or 21 September as well, to show the shoulder months. Where a scheme sits beside allotments, a productive growing season plot from late spring is sometimes requested. Where an existing solar array is affected, midwinter shading matters far more than the equinox, and the assessment should be run for December with hourly detail around solar noon.

Ask at pre-application which dates the authority expects. It costs one email and it avoids a second modelling exercise, which is the kind of avoidable cost our note on phasing a survey budget across two financial years is written to prevent. The same principle applies to work whose validity depends on the time of year, as with winter ground investigation.

And one honest limit. These dates and criteria come from guidance, not from statute. BRE 209 says its numerical values are advisory and may be varied where circumstances warrant, and inspectors have accepted departures in dense urban settings where the alternative is no development at all. Report the guidance figures first, then argue the departure, in that order.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Daylight, sunlight and overshadowing modelling is specialist work, so a suitably qualified author and professional review are strongly advised.

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