Sunlight to public squares, parks and play space

Sunlight to public open space is tested on 21 March, and the criterion most authorities apply is that at least half the area should receive at least two hours of direct sunlight on that date. If development reduces the compliant area to less than about eight tenths of its former extent, the loss is treated as noticeable. Those two numbers carry most sunlight public open space assessment arguments, and a committee can follow them without any technical background.

What they do not capture is how a square feels. A plaza that scrapes past the criterion while sitting in a wind-scoured shadow at the base of a tower has passed a test and failed as a place.

What the test measures for open space

The assessment models direct sun reaching the ground plane, taking account of the proposed buildings, the existing surroundings and any retained structures. It reports the proportion of the space receiving two or more hours of sun on 21 March, before and after the development.

The reference date matters. The equinox sits between the summer condition, when almost anything passes, and the winter condition, when almost nothing does. It is the fair middle case, and it is the date the guidance names.

The two-hour threshold is about usability rather than about brightness. Two hours of sun is roughly what a space needs for people to sit, for a cafe to put tables out, for grass to establish. Below that the space becomes a route rather than a destination.

Model the ground plane, not a horizontal plane at some arbitrary height. Steps, terraces and raised planters all change the result, and a square with a sunken centre performs worse than its plan suggests.

The proportion receiving sun on the reference date

Two figures are reported and both matter.

The absolute figure is the proportion of the space meeting the two-hour test after development. The relative figure is that proportion as a fraction of the pre-development value. A space already heavily shaded by existing buildings may fail the absolute test in the baseline, in which case the relative comparison is the meaningful one.

Be careful about the baseline. Where a site is currently cleared or occupied by low buildings, the baseline sunlight is unusually generous, and a scheme of ordinary density will show a large relative loss against it. Whether that is a fair comparison depends on what the plan allocates the site for, and the choices involved are the same ones discussed in mirror image baseline and existing obstructions.

Report the numbers for each space separately. Averaging across a masterplan hides the one courtyard that gets no sun at all, and consultees find it.

Play space and its particular sensitivity

Children's play space is the receptor that attracts most weight, and there are good reasons for it.

Play space is used in the middle of the day, when the sun is highest and a shadow from a tall building is at its shortest, so a space that fails the test is failing at the best time rather than the worst. Younger children use it in colder months when sun matters more for comfort. And play space provision is usually a policy requirement calculated from the child yield of the scheme, which means a space that is unusable is a policy failure as well as an amenity one.

Where a scheme provides play space to meet a quantitative standard, show that the space assessed for sunlight is the same space counted towards the standard. Schemes that meet the area requirement with a strip along the north side of a block, and then assess sunlight on a different courtyard, are picked apart at consultation.

Doorstep play for the youngest age group is the hardest to place well, because it needs to be close to homes, overlooked, and sunny, and the three requirements compete. The Play Space Assessment should set out how that was resolved rather than presenting the outcome as inevitable.

Wind and sun together in the same space

Sunlight and wind are assessed by different consultants and reported in different documents, and the interaction between them is where the quality of a space actually sits.

A space with good sun and severe wind is unusable for sitting. A sheltered space in permanent shade is unusable in winter. The comfort criteria used in wind assessment classify locations by activity, from sitting through standing and strolling to walking, and the categories map neatly onto what a landscape design intends each part of a space to be.

Run the two assessments against the same massing model and the same landscape layout, then overlay them. The result usually shows that the sunny parts of the space and the sheltered parts are different parts, which is a design problem worth discovering before the application rather than at committee.

Mitigation that helps both: canopies and colonnades at the base of tall buildings, tree planting positioned to break wind without casting shadow across seating, setbacks at podium level, and locating the seating where the overlay shows both criteria are met.

A Wind Microclimate Assessment is required on most tall building schemes, and coordinating it with the sunlight work costs nothing if it is arranged at the outset.

Seasonal plots that help a committee

Numbers convince technical consultees. Images convince members.

Produce shadow plots at hourly intervals on 21 March, 21 June and 21 December, rendered over the landscape layout rather than over a bare site plan so the seating, play equipment and planting are visible. Add a cumulative sun hours plan for 21 March, coloured by the number of hours each part of the space receives.

That last drawing is the single most useful output. It shows immediately which bench gets sun at lunchtime and which corner never does, and it lets a member form a view without reading a table. The wider presentation principles are set out in presenting daylight results a committee can read.

Where a courtyard is the space in question, the geometry behaves in ways that surprise people, and the analysis in courtyard blocks and sunlight to shared amenity space applies directly.

Design responses in a tall building scheme

The responses that work, roughly in order of effectiveness:

1. Move the tall element to the north side of the space, so its shadow falls onto the street or onto lower buildings rather than across the square.
2. Slim the tower and accept the same floorspace in a taller, narrower form, since a slender shadow sweeps across quickly while a wide one dwells.
3. Step the massing down towards the space, which also improves the wind condition at ground level.
4. Shift the space itself within the site, which is often easier at masterplan stage than moving a building.
5. Split the provision, so that a smaller sunny space and a larger sheltered one together serve different uses.

What does not work is arguing that the shadow moves. It does, and so does the criterion, which already accounts for it by measuring hours rather than instantaneous coverage.

Reporting the result

A clear report covers the assessed spaces on a plan with references, the method and software, the baseline and proposed figures for each space in a single table, the hourly plots at the three reference dates, the cumulative sun hours drawing, and an explicit statement where a space falls below the criterion.

Include the honest case. A dense urban scheme will usually have one space that underperforms, and identifying it, explaining why it is acceptable, and showing what was done to limit the effect reads far better than a report claiming that everything passes.

Where the space forms part of a wider open space provision calculation, the Open Space Assessment and the sunlight study should refer to the same areas and the same boundaries. Two documents drawing the space differently will both be queried.

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.