Daylight modelling dates: 21 March, June and December

Daylight assessment has no date. Sunlight assessment does, and the date it uses is 21 March. That distinction explains most of the confusion about daylight sunlight assessment dates, because the two halves of the same report work on completely different physical assumptions.

Daylight is modelled under a standard overcast sky, which has no sun in it, so the time of year is irrelevant and the result is the same in January as in July. Sunlight needs a solar position, which means a date, a latitude and a time of day.

Daylight has no date, sunlight does

The overcast sky model used for daylight is a mathematical construct with a defined luminance distribution and no directional sun. Vertical sky component, the no sky line and the illuminance-based measures introduced into the current British and European daylight standard all work within that model. None of them takes a date as an input.

So when a council asks for daylight modelling on the equinox, it is usually asking for the sunlight and overshadowing work rather than the daylight calculations. Answer the question they meant, and say in the method section which measures are date-dependent and which are not. That one paragraph prevents a round of pointless consultation comments.

Why 21 March is the reference date

The spring equinox is the point at which day and night are close to equal and the sun's path is roughly halfway between its summer and winter extremes. Using it gives a condition that sits in the middle of the annual range rather than at either end, which is why the published good practice guidance on site layout for daylight and sunlight adopted it for the amenity space test.

A detail worth knowing. The astronomical equinox falls on 20 March in most years, and the guidance nonetheless uses 21 March as its convention. The difference in solar altitude between the two dates is negligible for these purposes, so model 21 March and label it as such rather than quietly substituting 20 March because your software defaults to the true equinox.

September gets overlooked. The autumn equinox around 21 September produces a near-identical sun path, so a scheme assessed on 21 March has effectively been assessed for the autumn condition too, and saying so heads off a request for a second run.

The sun hours on ground test

The amenity space test is simple to state and easy to get wrong in the modelling. The guidance recommends that at least half of an amenity area should receive at least two hours of sunlight on 21 March, and the same figure is applied to assess whether a new building makes an existing garden or open space materially worse.

Four modelling decisions change the answer.

1. The extent of the amenity area assessed. A rear garden assessed to its full depth behaves differently from one assessed only over the usable part, and a communal courtyard assessed including the circulation strip differently again. State the boundary used and show it on a plan.
2. The ground plane height. The test is at ground level, so a podium garden has to be modelled at podium level rather than at street level.
3. What is included as an obstruction. Existing buildings, the proposed building, boundary walls and fences, and mature trees where they are evergreen. Deciduous trees are normally excluded with a note, because they are leafless in March.
4. The time step. A calculation on a coarse time step can miss short periods of direct sun that a fine step picks up, and the aggregate two hour figure shifts with it.

Annual probable sunlight hours and the winter figure

Sunlight to windows uses a different measure. Annual probable sunlight hours expresses the proportion of the total hours of sunlight that a window can expect, taking into account the local climate, and the guidance applies it to windows facing within 90 degrees of due south.

The recommendation has two limbs. A window should receive at least 25 per cent of annual probable sunlight hours, including at least 5 per cent during the winter months, with that winter period defined as 21 September to 21 March. A reduction to below 0.8 times the former value, where the loss also exceeds 4 per cent of annual probable sunlight hours, is treated as a noticeable loss.

The winter limb is the one that decides contested cases. A tall building to the south of an existing terrace can leave the annual figure comfortable while removing almost all of the winter sun, and an assessment that reports only the annual number has hidden the effect. Report both limbs for every window assessed, and the detail of how the measure is built up is set out in annual probable sunlight hours explained.

Solstice studies and what they add

Neither solstice features in the recommended tests, and both are useful. 21 June shows the best case, which matters for overheating and for shading of glazed facades, and it is the date on which a courtyard scheme looks most attractive in a committee report. 21 December shows the worst case, with the lowest solar altitude of the year and the longest shadows.

Use them as context rather than as a test. A set of hourly shadow plots on 21 March, 21 June and 21 December gives a reader the annual range in three sheets, and an officer deciding whether a communal garden will be usable finds that more persuasive than a single percentage.

And keep the two purposes separate. Solstice plots illustrate. The 21 March calculation assesses. An Overshadowing Study that presents June plots and no March calculation has produced a brochure.

Choosing dates for an amenity space study

For a residential scheme with a communal garden, the defensible set is 21 March for the two hour test, hourly transient plots on 21 March, and hourly plots on 21 June and 21 December for context. Add 21 September only where the space is used seasonally and the autumn condition is in dispute.

For a children's play space, consider the times of day as much as the dates. A space in full sun on 21 March between 10am and midday is of limited use to a school-age child, so plot the late afternoon period separately. For an outdoor seating or restaurant terrace, the commercially relevant period is the evening in summer, which is a 21 June question rather than an equinox one.

Where the space is a private garden belonging to a neighbour rather than part of your own scheme, stay with the recommended test. Introducing additional dates to improve the appearance of your own numbers reads exactly as it is.

Presenting the date basis clearly

Six items to state on the face of the assessment.

The guidance document and edition relied on. The dates modelled. The time step and the period of day covered. The latitude or location data used, since solar position varies across the country and a London sun path applied in Newcastle is wrong by a measurable amount. The obstructions included and excluded, with trees dealt with explicitly. And the ground or reference plane height for each area assessed.

Put that list in a table at the front of the Daylight and Sunlight Assessment rather than scattering it through the appendices. Reviewers check it first, and a complete table buys credibility for everything after it.

Where a council asks for more dates

Some authorities ask for additional dates, particularly in dense urban contexts where amenity space is scarce and heavily used. Requests for a May or August condition, for term-time afternoon periods, or for a full annual analysis all turn up.

Deal with them in the design rather than in the argument. If a courtyard fails at 21 March, adding an August plot showing it passes will not persuade anyone, whereas moving a single storey off the southern block might. The design response belongs in the Design and Access Statement, and the numbers belong in the technical report.

Illustrative example: a five storey infill block on the northern edge of a courtyard, where the 21 March calculation gives 46 per cent of the courtyard with two hours of sun, just below the recommended figure. Setting the top floor back 3 metres and widening the gap between blocks takes it to 54 per cent. That is a drawing change found by modelling at the right date early, which is the whole point of knowing which date to use.

Browse the 66 report types to see which amenity documents your scheme needs, and have the daylight and sunlight work prepared or reviewed by a suitably qualified consultant.

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