Daylight modelling at the equinox and the other dates

Sunlight to gardens and open spaces is assessed on 21 March. That single date does most of the work in a daylight and sunlight report, because 21 march daylight modelling gives the equinox condition, when day length is roughly equal everywhere and the sun path is the average of the year rather than a seasonal extreme. The BRE guidance on site layout planning for daylight and sunlight uses it as the reference date for the sun on ground test, and officers expect to see it.

The other two dates, 21 June and 21 December, are supporting information. They answer different questions, and presenting them as alternatives to the equinox result is the quickest way to confuse a committee.

Why the equinox is the assessment date

At the equinox the sun rises due east and sets due west, and the altitude at midday is set by latitude alone. That makes the test comparable between sites in a way a midsummer or midwinter test is not.

The BRE recommendation for amenity space is that at least half of the area should receive at least two hours of sunlight on 21 March. Where an existing space is affected by new development, the guidance also looks at whether the sunlit area is reduced below a proportion of its former value. Those are guidelines rather than standards, and the guidance itself says the numerical values should be applied flexibly according to context, which matters in dense urban schemes where meeting them is not realistic.

Report the equinox result first, clearly, as the headline. Everything else supports it.

What 21 June and 21 December add

Midsummer shows the best case. It is useful for demonstrating that a courtyard or a roof terrace works as amenity space in the season it will actually be used, and it is worth presenting for schemes whose case rests on the quality of communal space.

Midwinter shows the worst case. Low sun produces long shadows, and a winter plot demonstrates the effect on south-facing windows, on solar panels, and on spaces that residents pass through rather than sit in. It is rarely determinative and it is often what an objector asks for, so producing it before being asked removes an argument.

Neither replaces the equinox. Where a report leads with a June plot showing a sunny courtyard and mentions March in an appendix, an experienced officer reads that as an attempt to flatter the scheme.

Daylight does not have a date at all

This is the distinction that causes most of the confusion at committee, and it is worth stating explicitly in the report.

Daylight assessment, using vertical sky component at a window and the no sky line inside a room, is calculated against a standard overcast sky model. There is no date, no sun position and no time of day involved. Sunlight assessment, using annual probable sunlight hours at a window and the sun on ground test for spaces, does depend on sun position and therefore on dates.

When a councillor asks why the daylight figures do not change between March and June, that is the answer. Put it in the methodology section in two sentences and you will not have to give it verbally.

Annual probable sunlight hours brings its own seasonal element. The assessment looks at the total across the year and separately at the proportion received in the winter months, so a window that gets its sunlight only in summer performs worse than the annual total alone suggests. The winter period used runs from late September to late March.

Transient shadow plots and how to present them

Shadow plots show the shadow cast at intervals through a chosen day, typically hourly. They are the most persuasive material in the report for a non-technical audience and the easiest to mislead with.

Present them on a consistent base plan at a stated scale with a north point, the same extents on every sheet, and existing and proposed conditions side by side on the same page. Label the date and the time on every plot. Where a scheme is compared against a consented baseline rather than the existing site, say so on the drawing rather than only in the text.

Model what casts shadow. Existing surrounding buildings, boundary walls and the proposed development, at their actual heights. Trees are usually omitted or modelled without leaves, because deciduous canopies vary through the year, and whatever you do should be stated.

The assumptions to state in the methodology

Six, and they take a paragraph:

1. The guidance edition used, and any local policy or supplementary document that sets different tests.
2. The date or dates modelled and the reason for each.
3. The baseline against which effects are measured: existing site, consented scheme, or both.
4. Which surrounding buildings were surveyed and which were taken from mapping data.
5. Ground levels and the survey they came from.
6. How windows have been treated where the internal room layout is unknown, which is common for neighbouring properties.

The fifth is the one that gets reports rejected. A model built on assumed levels on a sloping site produces results nobody can rely on, and a topographical survey at the outset is the less costly fix.

Where a policy sets its own dates

Some authorities, particularly in London, publish supplementary guidance setting additional requirements: extra dates, hours of sunlight to a defined proportion of a space, specific tests for child play space, or a requirement to model a future baseline including allocated sites. Read the local document before scoping the work, because a report that meets the national guidance and misses a local test has to be redone.

Where local guidance and the BRE approach differ, present both. A table comparing the result against each test is easier for an officer to use than two separate narratives.

Presenting the results for committee

Lead with the equinox sun on ground result for each relevant space, then the window tests, then the supporting dates. Give one clear table per receptor with the test, the target, the result and whether it passes, and reserve the discussion for the cases that do not. A Daylight and Sunlight Assessment that buries three failures in a 90 page appendix will have those three failures read out at committee by somebody else.

Where an existing neighbour is affected, say what the effect means in ordinary terms. A reduction in vertical sky component below the guideline at a kitchen window on a north elevation is a different matter from the same reduction at a living room window facing a garden, and the report should say which rooms are affected and how they are used.

An Overshadowing Study covering the amenity spaces, and a Wind Microclimate Assessment where the scheme is tall enough to need one, are usually commissioned from the same model, so scope them together and save the survey cost.

Browse the 66 report types to see how the amenity evidence sits with the rest of a submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. 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.