Two hours of sunlight on 21 March for amenity areas
BRE 209 asks whether half of an amenity area gets two hours of sun on 21 March, with a secondary check against 0.8 times the former sunlit area. Here is how to apply both.
The sunlight test for gardens and amenity areas in BRE 209 asks whether at least half of the area receives at least two hours of direct sunlight on 21 March. If a space fails that test, there is a secondary check: whether the sunlit area has been reduced to less than 0.8 times its former value. A space that meets the half-area test passes. A space that fails it but retains more than 80 per cent of its previous sunlit area is usually still reported as acceptable, and a space that fails both is where the amenity argument begins.
That is the whole mechanism. The difficulty is applying it to spaces BRE 209 was not drafted around.
The test for sunlight to gardens and open spaces
BRE 209, Site Layout Planning for Daylight and Sunlight, deals with amenity space separately from windows because the question is different. A window is assessed for annual probable sunlight hours. An outdoor space is assessed for whether people can sit in the sun in early spring.
The assessment is a shadow study. The modeller builds the proposed massing and the surrounding context, runs the sun path for 21 March, and records for each point in the amenity area the total duration of direct sun across the day. The output is a plan of the space divided into areas receiving two hours or more and areas receiving less.
Both the existing and proposed conditions have to be modelled where an existing space is affected. Without the existing case there is no 0.8 comparison and the secondary test cannot be applied, which is a frequent omission in reports prepared quickly.
Why 21 March is used as the assessment date
The spring equinox is the midpoint of the year for solar geometry, which makes it the honest date to assess.
Choose 21 June and almost everything passes, because the sun is high and the day is long. Choose 21 December and almost everything fails, because the sun barely clears the surrounding roofs in a British city. The equinox gives a result that correlates reasonably with usable amenity across the year, and because it is fixed, schemes can be compared.
It also carries a practical consequence. The sun altitude at the equinox is roughly equal to 90 degrees minus the site latitude, so a scheme in Newcastle has a lower equinox sun than the same scheme in Southampton and the shadows are longer. The same building form produces different results in different parts of the country, which is worth remembering before copying a precedent from another city.
The half-of-area and 0.8 times former tests
The two tests work in sequence, and reports that present only one of them are incomplete.
| Result | Reported position |
|---|---|
| At least half the area gets two hours of sun on 21 March | Meets the BRE guidance |
| Less than half, but sunlit area is at least 0.8 times the former value | Loss likely to be acceptable |
| Less than half and below 0.8 times the former value | Noticeable loss, needs justification |
The thresholds are guidance, not policy, and BRE 209 says so in its own introduction. The numerical result is the starting point for a planning judgement about the character of the area, the existing quality of the space and whether the design could reasonably do better. An officer in a dense inner London borough weighs a failure differently from an officer in a low-density suburb, and the Daylight and Sunlight Assessment should say which context it is arguing in.
Which spaces the test applies to
BRE 209 points the test at the main area of amenity used by the occupants: back gardens, communal courtyards, children's play areas, sitting-out areas, and public open space where sunlight matters to its function.
It is not meant for every patch of grass. A verge, a service strip, a narrow side passage and a landscape buffer with no amenity function are not amenity areas, and assessing them inflates the number of failures without telling anyone anything. Equally, selecting only the sunniest part of a garden as the assessed area is the trick officers look for. Define the space on a plan, state why that boundary was drawn, and keep it consistent between the existing and proposed cases.
For a terraced back garden, the convention is to assess the whole garden rather than a notional sitting area, because the occupant chooses where to sit.
Assessing podium gardens and roof terraces
Podium and roof-level amenity space is where the guidance runs out, and judgement takes over.
A podium garden enclosed by the scheme's own blocks is self-shaded, and in a courtyard form the lower levels often fail the half-area test comfortably. The useful analysis is not a single pass or fail but a map: which parts of the podium get two hours and whether the social functions, the seating, the toddler play area, the growing beds, have been placed in those parts. A scheme that locates its play space in the permanently shaded northern corner has made a design error that no amount of reporting fixes.
Roof terraces usually perform well and need little analysis, with two exceptions. Terraces overshadowed by a taller element of the same building, and terraces where a plant enclosure or lift overrun casts a shadow across the usable area. Model the plant. It is routinely left out of the massing model and it is routinely the thing that shades the seating.
Where the scheme relies on roof amenity to compensate for a failing podium, say so explicitly and show both.
Where tree canopy is counted
BRE 209 does not ask you to model existing trees as opaque obstructions, and in general shadow studies for sunlight to amenity areas exclude vegetation.
The reasoning is that deciduous canopy is in leaf for part of the year only, that trees can be pruned or removed, and that tree shade is generally regarded as a benefit in an amenity space rather than a harm. On 21 March most deciduous species are not in leaf at all.
There are two situations where trees should be discussed in the text even if they are not in the model. A dense evergreen screen, a mature conifer belt or a yew hedge shades year-round, and reporting a garden as sunlit when a 12m Leyland cypress hedge stands on its southern boundary is misleading. And where trees are to be removed by the scheme, the existing case should not take credit for shade that is about to disappear. Where protected trees are involved, the tree survey to BS 5837:2012 establishes what is being retained, and that should match the massing model.
Presenting shadow plots
Four outputs make a report checkable, and a reviewer will ask for them if they are missing.
- A plan showing each assessed amenity area, outlined and labelled, in both the existing and proposed cases.
- Hourly shadow plots across 21 March, typically from an hour after sunrise to an hour before sunset, so the pattern of movement is visible rather than a single snapshot.
- A two-hour sunlight contour plan for each space, shaded to show the compliant area, with the percentage stated.
- A results table giving existing area, proposed area, percentage of the space meeting two hours, and the proposed-to-existing ratio.
State the model assumptions alongside them: the ground datum, the context buildings included and their source, whether balconies and plant were modelled, and the software used. A report that presents percentages without the model description cannot be verified, and a reviewer who cannot verify it will treat the numbers cautiously. The same discipline applies to window analysis, covered in daylight modelling software outputs and what to include, and the distinction from the civil law position is set out in rights of light contrasted with planning amenity conditions.
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Frequently asked questions
What is the two hour sunlight test for gardens?
BRE 209 asks whether at least half of an amenity area receives at least two hours of direct sunlight on 21 March. If it does, the space meets the guidance. If it does not, the secondary test is whether the sunlit area remains at least 0.8 times its previous value, in which case the loss is usually reported as acceptable.
Why is 21 March used rather than midsummer?
The spring equinox is the midpoint of the solar year, so it gives a result that correlates reasonably with usable amenity across the seasons. Midsummer would pass almost any scheme because the sun is high and the day long, and midwinter would fail almost everything. A fixed date also lets schemes be compared with one another.
Do existing trees get modelled in a sunlight study?
Normally not. Deciduous canopy is absent on 21 March, trees can be pruned or removed, and tree shade in an amenity space is generally treated as a benefit. Dense evergreen screens are worth describing in the text, and an existing case should not claim credit for shade from trees the scheme will remove.
How are podium gardens assessed?
With the same test, but a single pass or fail tells you little because courtyard forms self-shade at lower levels. The useful output is a map of which parts of the podium receive two hours, checked against where the seating, play space and growing areas are placed. A play area in a permanently shaded corner is a design problem, not a reporting problem.
Is the BRE guidance a policy requirement?
No. BRE 209 states that its numerical thresholds are guidance to be applied flexibly, and the planning decision weighs them alongside the character of the area, the existing quality of the space and whether a better design was reasonably available. Some local plans and the London Plan give the guidance more weight, so check the adopted policy.
Which outputs should a sunlight report include?
A plan outlining each assessed amenity area in existing and proposed cases, hourly shadow plots across 21 March, a two hour sunlight contour plan for each space with the compliant percentage stated, and a results table giving the proposed to existing ratio. Add the model assumptions, including context buildings, balconies, plant enclosures and the software used.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.