Transient overshadowing of a children's play area

Play area overshadowing is tested on sun reaching the ground, not on sky reaching a window. That one difference changes the whole method. There are no vertical sky components, no no-sky lines and no window reference points. What matters is how much of the playable surface gets how many hours of direct sun on a stated date, and whether the proposal reduces it.

The benchmark most authorities use comes from the BRE site layout guidance: at least half of an amenity area should receive at least two hours of sunlight on 21 March, and where an existing sunlit area is reduced to less than 0.8 times its former extent the loss will be noticeable. Those two figures do most of the work in a play space case.

Why play space is tested on sun on ground

Children's playable space is an outdoor surface that people occupy. Its quality depends on whether it is warm and bright enough to use, and sun on ground measures that directly.

The window-based daylight criteria answer a different question about interiors, and applying them to a play area produces numbers that mean nothing. Officers notice. An assessment that reports vertical sky component figures for a play space has told the reader it used the wrong method.

There is a policy reason too. Play space is usually required by a quantitative standard, commonly expressed as an area per child derived from the development's child yield, and the standard assumes the space is usable. A play area in permanent shadow meets the area requirement on paper and fails it in substance, which is why authorities ask for the sun on ground test alongside the quantum calculation in a Play Space Assessment.

The equinox benchmark and the two solstices

Three dates are conventionally modelled, and each answers something different.

21 March, the spring equinox, is the assessment date in the BRE guidance. Day length is roughly twelve hours and the sun path is intermediate between the extremes, so it covers the shoulder seasons when outdoor space is most sensitive to shading.

21 June, the summer solstice, shows the best case. The sun is highest and the shadows are shortest, so a play area that fails in June is in serious trouble.

21 December, the winter solstice, shows the worst case. Low sun angles mean long shadows and in a dense scheme much of the ground is in shade all day. Reporting December is useful context but it should not be the test, because almost nothing passes.

Model all three and report the equinox against the criterion. Where a scheme is marginal on 21 March, adding a second shoulder date in September adds nothing, since the sun path is nearly identical.

Transient shadow against annual shadow

Two different outputs come from the same model and they are routinely confused.

A transient shadow study shows the shadow footprint at discrete times, typically hourly from sunrise to sunset on a given date. It is a sequence of plan diagrams and it shows where the shadow moves. It is useful for design, because it reveals which building edge is responsible for the shading at the times that matter.

A sun hours on ground analysis aggregates the same information into a single plan showing, for every point on the surface, how many hours of direct sun it receives on that date. It is the output the criterion is measured against, because the test is about duration rather than instantaneous coverage.

A permanent shadow or year-round shade plot identifies the parts of the site that never receive direct sun on the modelled date. For a play space that is the most damaging finding, and it is worth running separately.

Submit the sun hours plot as the primary evidence and the transient sequence as supporting material. Submitting only the transient diagrams invites the officer to count hours off a series of drawings, which nobody enjoys doing.

Proportion of area in sun and the threshold used

The calculation is straightforward once the model is built:

1. Define the play area boundary precisely, excluding paths, planting beds and any surface that is not playable, because including them inflates the result.
2. Compute sun hours on ground across that area for 21 March, using a surface grid fine enough to resolve the shadow edges.
3. Report the percentage of the area receiving two or more hours.
4. Where an existing amenity area is affected, report the existing and proposed sunlit areas and the ratio between them.
5. State the criterion applied and its source.

Two places where results get massaged, usually unintentionally. The boundary definition, where a generous reading that includes an adjacent lawn can lift the percentage by ten points or more. And the assessment plane height, because testing at 1.5 metres above ground rather than at ground level shortens every shadow and makes the numbers better. Test at ground level and say so.

Where the scheme fails, give the shortfall honestly in both measures. A play area with 38 per cent of its surface achieving two hours, against a criterion of 50 per cent, is a specific fact an officer can weigh. A statement that the area is "generally well lit" is not.

Relocating play space within a masterplan

Where the numbers fail, the first move is to put the space somewhere else. On a multi-block scheme that is usually possible and usually cheaper than reducing the massing.

The options that work, roughly in order:

• Move the space to the south or south-west side of the tallest block, so the building is behind the play area rather than in front of it.
• Shift it away from the base of the tallest element, since the shadow length scales with height and a play area at a block corner catches sun from two directions.
• Split it. Two smaller doorstep spaces in different parts of the scheme will often both pass where one large central space fails, and doorstep play for younger children is better close to homes anyway.
• Use a podium or roof level above the shadow line, accepting the access and supervision implications.
• Reduce the height of the single element causing the shading, which on close inspection is often one stair core or lift overrun rather than the whole block.

What does not work is enlarging the space. The criterion is proportional, so a larger area in the same shadow fails by the same percentage.

Where allotments, growing space or orchards form part of the open space strategy, the thresholds are different again and the growing season matters more than the equinox, as set out in overshadowing of allotments, orchards and growing space.

Planting and future canopy shadow

Trees are the omission that undermines otherwise careful studies.

Most models are built with buildings only, and the result is reported as compliant. Then the landscape drawings show a row of semi-mature trees along the southern edge of the play area, reaching 12 metres at maturity, and the sunlit proportion at year 20 is nothing like the figure submitted.

Run the analysis twice: with planting as proposed at installation size, and with planting at a stated maturity, usually taken at 15 to 20 years. Model canopies as a porous volume rather than a solid block, since deciduous canopies transmit some light and are bare in March in any case, which is one reason the equinox date is forgiving of deciduous planting and not of evergreens.

Then design around it. Evergreen planting on the southern edge of a play space is the combination to avoid. Deciduous trees on that edge give shade in July, when it is wanted, and let light through in March, when it is needed.

Presenting the study

An overshadowing submission that gets accepted contains:

A plan of the assessed area with its boundary dimensioned and the excluded surfaces identified. Sun hours on ground plots for 21 March, 21 June and 21 December, with a legend in hours and the assessment plane stated. A table giving the percentage of the area achieving two or more hours on each date, with the criterion alongside. Transient shadow diagrams at hourly intervals on 21 March. A permanent shade plot. The planting scenario, at installation and at maturity. And a short statement of the model inputs, including the geographic location, the north point, the time basis and the surrounding context buildings included.

Context buildings are worth a sentence of their own. A study that models only the application site will overstate the sunlight where neighbouring buildings cast shadow onto the play area, and the officer will ask for it to be rerun.

Put the analysis in an Overshadowing Study rather than appending it to the window-based daylight work, because the methods and the criteria are different and mixing them makes both harder to check. Councils expect a suitably qualified author for this work and professional review of any draft is strongly advised.

How much weight a council gives the criterion varies, and the drafting of the local policy is what decides it, which is the point made in local plan amenity policies and how they use BRE 209. Some plans incorporate the figures as standards and some reference them as guidance, and the difference changes whether a 38 per cent result is a refusal reason or a matter for the planning balance.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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