Sun-on-ground and transient overshadowing methods compared

Two different pieces of analysis get filed under the same heading. Sun-on-ground testing measures how much of a garden, courtyard or play area receives at least two hours of sunlight on a single reference date. Transient overshadowing plots the moving shadow of the proposed building across a sequence of hours and dates. They answer different objections, and the choice of overshadowing assessment methods should follow what the council and the neighbours are actually complaining about.

If the objection is that a garden will stop being usable, the sun-on-ground test is the one with a numerical threshold behind it. If the objection is that a shadow will fall across a terrace at four o'clock on a summer afternoon, only a transient study will show it.

What each method measures

Sun-on-ground analysis, sometimes called sunlight availability to amenity space, comes from the amenity chapter of BRE 209, Site layout planning for daylight and sunlight, in its 2022 edition. It produces a plan of the assessed area shaded to show which parts get two or more hours of direct sun on 21 March, expressed as a percentage of the total area.

Transient overshadowing produces a set of shadow outlines at intervals through a day, usually hourly, for chosen dates. It is descriptive rather than criterion-based. There is no pass mark in BRE 209 for a transient sequence, which is both its strength and its weakness: it shows exactly what happens and leaves the planning judgement to the reader.

Councils ask for them in different circumstances. A residential scheme with a new communal garden or a children's play area will normally need the sun-on-ground test, because there is a standard to measure against. A tall building next to an existing public square, a school playground or a cluster of private gardens will usually need the transient sequence as well, because that is what the objectors will describe.

The two-hours-on-21-March test

The BRE guidance suggests that at least half of an amenity area should receive at least two hours of sunlight on 21 March. Where an existing space already falls short and a new building makes it worse, the further guideline is that the loss becomes noticeable if the sunlit area after development is less than 0.8 times its former value.

Two things about the date. It is the spring equinox, chosen because it sits midway between the extremes and gives a defensible average condition. And it is deliberately not 21 June, because a midsummer test flatters almost any scheme.

The two hours do not have to be continuous. That matters on courtyard schemes where a gap between blocks admits sun in two separate windows during the day, and it is worth stating in the report, because a reader who assumes continuity will misread the plot.

Modelling conventions to record in the method section:

• the sun path model used and the site latitude and longitude
• whether existing trees and boundary walls are included in the model
• the height at which the test plane sits, which is normally ground level
• which parts of the amenity area are excluded, such as paths, bin stores and parking
• surrounding buildings retained, demolished or consented but unbuilt

That last point produces most of the disputes. A scheme assessed against the existing surroundings can look very different from the same scheme assessed against the surroundings plus an adjacent consented tower.

Building a transient shadow sequence

Pick dates that bracket the year rather than flatter the scheme. The usual set is 21 March, 21 June and 21 December, with 21 September added where the client wants symmetry checked. Hourly intervals from an hour after sunrise to an hour before sunset are standard, and half-hourly intervals are worth using where a shadow crosses a small target space quickly.

Present the results as a plan sequence, then as a summary that names the affected receptors and the periods when each is in shadow. A sequence of 30 unlabelled diagrams is not an assessment. The narrative should say, in plain words, which gardens lose sun and between which hours.

December sequences deserve care. Almost every scheme performs badly on 21 December, and so does the existing situation, so the honest presentation is a before and after pair rather than the proposed condition alone.

Reading a 50 per cent result and its limits

A result showing 52 per cent of a garden meeting the two-hour test is compliant and, on its own, tells you very little. The figure is an area percentage, so a space can pass while the part people actually use, the bit next to the back door with the table on it, sits in permanent shade.

Report the distribution as well as the number. Where the sunlit area concentrates in a strip along a boundary that nobody can furnish, say so. BRE 209 is explicit that its numerical guidelines are advisory and should be applied flexibly according to context, and an inspector reading a scheme in a dense urban setting will expect that flexibility to be argued rather than assumed.

The other limit is what the test cannot see. It says nothing about wind, nothing about privacy and nothing about the quality of the space. A courtyard with excellent sun and a wind downdraught from the tower above it is not a usable garden, which is why a Wind Microclimate Assessment is often commissioned alongside on taller schemes.

Illustrative example: a communal garden

Illustrative example. A four-storey block is proposed on the northern edge of an existing communal garden serving flats. The baseline sun-on-ground plot shows 64 per cent of the garden meeting the two-hour test on 21 March. With the new block modelled, that falls to 56 per cent. The scheme still passes the 50 per cent guideline, and the ratio of new to former sunlit area is above 0.8, so the loss is within the further guideline as well.

The transient sequence tells the second half of the story. The new shadow sweeps across the seating area between roughly 9am and 11am in March and clears it by midday, while the summer condition is barely changed. That is a very different conversation with residents than a shadow arriving at 4pm, and it is the sort of finding that supports a modest relocation of the seating rather than a redesign of the block. These figures are illustrative and are not taken from a real scheme.

Presenting both methods in one study

An Overshadowing Study that carries both methods should keep them clearly separated: method, results, judgement, in that order, with the criterion-based test first because it is the one with a threshold. Where the same model has produced window-based results, the daylight and sunlight work belongs in a Daylight and Sunlight Assessment rather than being folded in, since the metrics, the receptors and the guidance sections differ.

Amenity space assessments interact with open space and play requirements, so where a scheme is also demonstrating child yield and playable space, use the same drawing of usable amenity area in both documents. Inconsistent area schedules between two consultants is one of the easiest things for an objector to spot.

Where rights of light differs from planning amenity

Rights of light is a private property right, usually acquired by 20 years of enjoyment under the Prescription Act 1832, and it is enforced through the civil courts with injunctions and damages. It uses its own conventions, based on internal illuminance at a working plane rather than area of garden in sun.

Planning has no power to resolve it. A council can grant permission for a building that later gets injuncted, and the officer report will often say as much in a single line. Advise clients to take separate rights of light advice early on any scheme in a tight urban block, because the remedy sits outside the planning system entirely.

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