Shadow analysis at the equinox (21 March)

BRE 209 uses 21 March — the spring equinox — as the benchmark date for testing overshadowing of amenity space. The test (the "open-space test") asks whether at least half the area of a garden, park, courtyard, play space or other amenity surface receives at least two hours of direct sunlight on that day, both before and after the proposed development. It is the equivalent of the VSC/APSH test for windows, but applied to open space. Many reports also include transient overshadowing plans: hour-by-hour snapshots of shadow on 21 March and sometimes on 21 June and 21 December, to illustrate how the shadow moves during the day.

Why 21 March?

The spring equinox is the mid-point between the summer and winter extremes of the UK sun path:

• On 21 March and 21 September, the sun rises due east and sets due west; day length is close to 12 hours.
• The maximum solar altitude in London at noon is approximately 38 degrees above the horizon.
• Shadows cast at the equinox are therefore longer than at midsummer (when the sun reaches over 60 degrees) but much shorter than at midwinter (when the sun peaks at about 15 degrees).

BRE 209 chose 21 March for three reasons:

1. Representativeness. Spring and autumn amenity use is the range in which the test is most meaningful; midwinter sun is too low to reach most urban gardens at all, and midsummer sun is too unobstructed.
2. Symmetry. Because the equinox sun rises due east and sets due west, the test is independent of the orientation of the amenity area — no single orientation is disadvantaged.
3. Comparability. Using a single fixed date produces numbers that are comparable between schemes, sites and boroughs. The seasonal variability is deliberately set aside.

The trade-off is that an equinox-only test can understate overshadowing that is disproportionately concentrated in one season. This is where transient overshadowing analysis, described below, comes in.

The BRE open-space test

The test is in BRE 209 section 3.3. In outline:

1. Identify the amenity area — the plan of the garden, park, courtyard or other surface being assessed.
2. Build a 3D model with the baseline surroundings (existing buildings, trees of significant size) and the proposed development.
3. For each hour of 21 March from sunrise to sunset, project the shadow cast by the massing onto the amenity ground plane.
4. For each point in the amenity area, count the number of unshaded hours it receives. A point that receives at least two hours of direct sun is "adequately sunlit".
5. Calculate the area of adequately sunlit amenity, as a percentage of the total amenity area, for both the baseline and proposed cases.

The BRE threshold: at least 50% of the amenity area should be adequately sunlit in the proposed case. Where the baseline already has less than 50%, the 0.8 ratio applies instead: the adequately sunlit area should not fall below 80% of its baseline extent.

Transient overshadowing plans

Alongside the open-space test, a BRE report typically includes transient overshadowing plans: hour-by-hour shadow diagrams showing how shadows cross the ground over the day. These are almost always drawn for 21 March, usually also for 21 June and 21 December to illustrate seasonal range.

The plans are read qualitatively:

• At what time of day does the proposal's shadow enter a given garden?
• Does it cross the whole amenity area, or only a strip?
• How long does the shadow remain on a specific point (e.g. a seating area, a children's play space)?

A shadow that sweeps across a garden for 45 minutes in the morning is materially different from one that sits on the same spot for four hours. The 2-hour test can hide that distinction; transient plans reveal it. For committee presentations and objections, the transient plans are often the most effective illustration.

Common pitfalls and challenges

• Amenity area not defined correctly. Garden boundaries should include useable soft landscaping; driveways, refuse stores, tarmac paths and other non-amenity surfaces should be excluded. Inflating the amenity area (counting the driveway) softens the loss.
• Tree canopies in the baseline. Large existing trees in the baseline model can shade amenity areas and reduce the baseline figure. If proposed trees are not in the baseline but the existing tall planes are, the numbers are not symmetric.
• Equinox only, transient missing. A report that presents the 2-hour equinox test but not the transient plans can hide severe morning or afternoon overshadowing. Always ask for the hour-by-hour plans.
• Children's play space treated as generic amenity. Play space has higher BRE expectations, and many Local Plans specify sunlight standards for LAPs, LEAPs and NEAPs (London Plan Policy S4). Check whether child-play amenity is separately assessed.
• Cumulative scheme used as baseline. As with VSC and APSH, a cumulative baseline including other consented schemes can reduce apparent loss.

How to challenge an overshadowing assessment

1. Identify the amenity area by name and reference the plan page in the report. "The communal garden serving Block B, 22 local road, shown on Plan 14 of Appendix 5."
2. State the numbers: existing adequately-sunlit area, proposed adequately-sunlit area, and both as a percentage of total amenity area. Show the arithmetic for the ratio.
3. Compare to the BRE thresholds: absolute 50%, ratio 0.8.
4. Describe the transient pattern: the hour at which the shadow enters the garden, the area affected, the duration. Refer to the hour-by-hour plans by timestamp.
5. Cite BRE 209 section 3.3 and the relevant local policy (London Plan Policies D6, G4, S4; borough amenity policies).
6. Name the specific use: "Residents use the garden for Saturday play groups between 10 and 12. After the development, the seating area will be in shadow for the whole of that window."

Frequently asked questions

Why is 21 March used for shadow analysis?
21 March is the spring equinox, the symmetric mid-point between midsummer and midwinter. It produces shadow patterns representative of spring and autumn use of amenity space, is orientation-neutral (sun rises due east, sets due west), and makes results comparable between schemes.

What is the BRE open-space test?
At least 50% of an amenity area should receive at least two hours of direct sunlight on 21 March. Where the baseline is already below 50%, the 0.8 ratio rule applies instead.

How is overshadowing calculated?
By tracking the sun hour-by-hour on 21 March and projecting each building's shadow onto the ground plane. The amenity area is scored by the number of sunlit hours received at each point.

What is transient overshadowing?
An additional hour-by-hour shadow analysis, usually drawn for 21 March (and sometimes 21 June / 21 December), showing how shadow migrates across amenity space over the day.

Sources