Garden overshadowing and the two hour sunlight test

If a neighbour's extension will shade your garden, or yours will shade theirs, the test a planning officer is most likely to reach for is simple: at least half of each garden or amenity area should get at least two hours of sunlight on 21 March. That criterion comes from the BRE guide on site layout planning for daylight and sunlight, not from planning law, and garden overshadowing planning decisions turn on how flexibly the officer applies it alongside the local plan's own amenity policy.

The test is about sunlight on the ground, which is a different thing from the sunlight tests applied to windows. A scheme can pass at every window and still take the sun off a small back garden, and that is the case most householder disputes are actually about.

The test applied to gardens and amenity areas

The BRE guidance asks you to look at each garden or amenity area as a surface and work out how much of it receives at least two hours of sun on the equinox date. Half the area is the benchmark. Where the garden already receives less than that, the guidance offers a second limb: the area that gets two hours of sun should not fall below 0.8 times its previous value.

That second limb does most of the work in practice. Plenty of urban gardens never met the half rule to begin with, because of orientation, boundary walls or the house itself, so the proportional test is the one that decides whether a change is acceptable.

Both limbs apply to private gardens, shared amenity space in flatted schemes, and spaces such as allotments and growing space where sunlight on the ground has a practical purpose. They do not apply to car parks, bin stores or circulation areas, and an assessment that includes those to inflate the compliant percentage is easy to see through.

Why 21 March is the assessment date

The equinox sits halfway between the extremes. On 21 March the sun follows a path close to the annual average, so a single day's analysis gives a reasonable picture of a typical year without flattering the scheme the way a midsummer date would.

Test the same layout on 21 June and almost everything passes, because the sun is high and shadows are short. Test it on 21 December and almost nothing does. Assessments occasionally include those dates as context, which is useful, and sometimes include them instead of the equinox, which is not.

One practical consequence: the shadow you see in your garden in July is not the shadow the assessment is based on. If you are objecting, photographs taken in high summer will not help you. March and late September photographs will.

Measuring the existing baseline first

The commonest failing in a submitted Overshadowing Study is that it reports the proposed condition without establishing the existing one. Without a baseline, the proportional limb cannot be applied at all, and the officer has no way of telling whether a 40 per cent result is a small change or a large one.

A sound assessment does four things in order.

1. Models the existing site and its surroundings, including boundary walls, fences at their actual height, and the host dwelling.
2. Reports the area of each garden receiving two hours or more of sun on 21 March before the development.
3. Repeats the analysis with the proposal in place.
4. States both figures and the ratio between them, garden by garden, rather than averaging across the site.

Trees are the awkward part. BRE guidance treats deciduous trees as partially transmitting rather than solid, and some assessors leave them out of the baseline altogether. Leaving them out makes the existing condition look sunnier than it is, which makes the proposed loss look worse. If you are reading someone else's assessment, check how the trees were handled before you accept the numbers.

Gardens already shaded by trees, walls or orientation

A north-facing garden behind a two storey terrace may receive two hours of March sun across a fifth of its area, and no extension anywhere will change that much. The proportional test then becomes the only meaningful measure, and the question is whether the proposal takes the remaining sunny strip or leaves it alone.

This is where the position of the extension matters more than its size. A single storey rear extension on the shaded side of a garden can have almost no effect on the sunlit area. The same extension on the sunny boundary can remove most of it. Moving a proposal 1.5 m along the rear elevation sometimes converts a failure into a pass, and that fix costs less than reducing the depth.

Hedges and fences cut both ways in an argument. A 2 m boundary fence that already shades the garden strengthens the applicant's case, because the baseline is low and the proportional change is small. It also means the officer is being asked to accept a garden with very little sun, which is where the local plan's amenity policy, rather than the BRE numbers, decides the outcome.

Where a reduction is acceptable in planning terms

BRE 209 says plainly that its figures are guidance to be applied flexibly, and planning decisions reflect that. A measured breach does not automatically refuse an application, and compliance does not automatically approve one.

Officers weigh the loss against the context. In a dense inner urban area, gardens are small and expectations are lower, and local plan policies often say as much. In a suburban estate with generous plots, the same percentage loss reads as more harmful because the comparison is with neighbouring gardens that keep their sun. The purpose of the space matters too, since a loss concentrated on the patio immediately behind the house is felt differently from a loss at the far end by the shed.

What rarely succeeds is arguing that the loss is acceptable because the garden is large. If the usable sitting-out area goes into shade, the total area is beside the point. Read the local plan amenity policies before you frame either case, because the way amenity policies use the guidance varies a great deal between authorities.

Evidence that persuades a case officer

For an applicant, the persuasive package is short. A plan showing each affected garden with the two hour contour before and after, a table of areas and ratios, a sentence on orientation and existing boundary features, and an honest statement where a garden falls short with the reason it is acceptable here. A Daylight and Sunlight Assessment that covers windows as well as ground sunlight is usually the right document when both are in issue.

For an objector, the persuasive package is different. State which garden, which part of it you use and when, what the existing March sunlight is, and why you think the submitted analysis is wrong. Point to a specific assumption, such as a fence modelled at 1.8 m that is actually 1.2 m, or a tree included that was felled last year. Officers can act on that. They cannot act on "it will block all our light", however true it feels.

And if the application has already been submitted, check whether any sunlight analysis was provided at all. On householder applications there often is none, and asking the council to require it is a reasonable request that sometimes changes the scheme before a decision is made.

You can see which reports a householder scheme is likely to need and draft the amenity sections from your own site details before commissioning the modelling.

This is general guidance, not legal advice. Amenity policies and the weight given to BRE criteria vary between councils, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.