Sunlight to a garden: the 21 March two hour test
Garden sunlight is assessed on 21 March, against a recommendation that at least half the amenity area receives two hours of direct sun. The before and after ratio decides most arguments on small urban plots.
Garden sunlight is assessed on one date. BRE 209, the guidance councils reach for when a neighbour objects to an extension on overshadowing grounds, recommends that at least half of a garden or other amenity area should receive at least two hours of direct sunlight on 21 March. If a new building reduces the sunlit area to below half, and to less than 0.8 times what it was before, the loss is likely to be noticeable to the people using the garden. That is the whole test in two sentences.
The two-season business belongs to windows, not gardens, and the confusion between them costs objectors their best argument.
What the garden test measures
The assessment is called sun on ground, and it measures area rather than hours at a point. A model of the site and its surroundings is built, the sun path for 21 March is applied, and the parts of the amenity area that receive at least two hours of direct sunlight across the day are plotted. The output is a percentage of the garden, before and after the proposed development.
Two numbers matter. The first is whether the sunlit area after development is at least half the amenity area. The second is the ratio of the after figure to the before figure, where 0.8 is the threshold BRE uses for a noticeable loss. Both are advisory, and BRE is explicit that the numerical guidelines should be applied flexibly according to context.
Which area counts is a judgement an assessor has to make and explain. The usable garden is the relevant space, not the whole curtilage. A front garden given over to parking, a side passage and the strip behind a garage are not amenity area in any meaningful sense, and including them to improve a percentage is the commonest way a sun on ground study loses credibility.
Why 21 March, and what the other dates are for
21 March is the spring equinox, when day length is close to twelve hours and the sun path sits roughly midway between its winter and summer extremes. It is a reasonable proxy for the shoulder seasons when sunlight in a garden is noticed and valued, and it avoids both the flattering midsummer case and the near-impossible midwinter one.
The other benchmark dates do different work. For windows, the annual probable sunlight hours test asks whether a main living room window receives at least 25 per cent of annual probable sunlight hours, including at least 5 per cent in the winter months between 21 September and 21 March. That is the genuine two-season assessment, and it applies to rooms, not to outdoor space. 21 June and 21 December are used for transient overshadowing studies and for solar panel and overheating work.
If an objection letter cites winter sunlight hours against a garden, an acoustician would call it a category error. The council's consultant will say so politely and the point will be lost. For a garden, argue 21 March.
Proportion of the garden, and the arithmetic of a small plot
The percentages behave unkindly on small urban gardens, which is why the flexibility in BRE 209 matters so much.
A north-facing garden behind a Victorian terrace, 6 m deep with a two-storey rear wall and party fences on both sides, may already receive two hours of March sunlight across well under half its area. A proposed extension next door then reduces an already failing figure. The before-and-after ratio becomes the operative number, because the absolute test is failed in the baseline and cannot be cured by anything the applicant does.
| Figure | What it tells you |
|---|---|
| Sunlit area after development, as a percentage of the amenity area | Whether the garden meets the recommended half |
| After area divided by before area | Whether the loss is likely to be noticed, with 0.8 as the marker |
| Baseline sunlit area | Whether the garden ever met the recommendation at all |
Where the baseline fails, a well-argued assessment says so plainly and then addresses the proportionate loss. Where the baseline passes comfortably and the proposal drops it below half, that is the strongest version of the argument. The way local plans pick up these figures varies a good deal, and local plan amenity policies and how they use BRE 209 is where the weight actually comes from.
Fences, outbuildings and the existing shadow
Baseline accuracy decides the result, and the baseline is usually wrong in the first draft.
A 1.8 m close-boarded fence on the southern boundary casts a band of shade roughly 2 m deep at the March equinox in southern England, varying with latitude and ground slope. That band is in the baseline, so it is already excluded from the sunlit area and the proposal cannot be blamed for it. The same goes for an existing garage, a mature conifer hedge, a neighbour's two-storey outrigger and the host dwelling itself.
Modelled heights have to come from somewhere defensible. A topographic survey with levels, or measured heights taken from a drawing set, are acceptable. Heights scaled from a photograph are not, and an assessment that does not say where its heights came from invites the obvious question.
Ground slope matters more than people expect. A garden rising away from the house gains sun. One falling away from a tall southern boundary loses it twice over, because the obstruction angle rises as the ground drops.
Where a loss is accepted, and where it is not
Councils grant permission for schemes that reduce garden sunlight every week. The guidance is advisory and amenity is one material consideration among many.
Acceptance is more likely where the context is dense and the existing condition already poor, where the loss affects part of a large garden rather than the whole of a small one, where the garden is one of several outdoor spaces serving the property, or where the affected strip is already in shadow for other reasons. Refusal is more likely where a scheme takes a compliant garden below the half mark, where the loss falls on the part of the garden immediately outside the main living space, or where the overshadowing comes on top of a separate daylight loss to windows.
Design changes that usually help: reducing the depth of a two-storey element while keeping the single-storey footprint, setting the upper floor back from the boundary, moving mass to the northern end of the plot, and switching a solid flank to a lower mono-pitch. Changes that rarely help: obscure glazing, which addresses overlooking and does nothing for sunlight, and planting, which adds shade.
Producing a shadow study that an officer can use
A shadow study is the practical output, and on a householder scheme it need not be elaborate. The requirement is that someone else can check it.
- Survey or measure the garden, the host dwelling, the proposed development and every obstruction within a sensible radius, with levels.
- Model the existing condition and the proposed condition from the same geometry.
- Plot sun on ground for 21 March, showing the two-hour contour on both scenarios, with the amenity area boundary marked.
- Give the before and after sunlit areas in m² and as percentages, and the ratio between them.
- State the assumptions: latitude, ground levels, obstruction heights and their source, and what you included as amenity area.
- Where the baseline already fails, say so in the text rather than burying it in a table.
On larger schemes this work sits inside a Daylight and Sunlight Assessment alongside the window tests, or as a standalone Overshadowing Study where gardens and amenity space are the only issue. Allotments, orchards and growing space are assessed on the same principles with a longer growing season in view, covered separately in overshadowing of allotments, orchards and growing space.
Rights to light is a different animal altogether. It is a private law matter about easements over defined apertures, it is not decided by the council, and a planning permission grants no immunity from it.
If you are preparing or responding to a householder application and the garden is the issue, enter the postcode on PlanScope to see which reports the site is likely to need before you commission a modelling exercise you may not require.
This is general guidance, not legal advice. 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.
Frequently asked questions
How much sunlight should a garden get under BRE 209?
The guidance recommends that at least half of a garden or amenity area receives at least two hours of direct sunlight on 21 March. Where development reduces the sunlit area below half, and to less than 0.8 times its previous extent, the loss is likely to be noticeable. Both figures are advisory and applied with regard to context.
Why is 21 March used rather than a summer or winter date?
It is the spring equinox, so day length is close to twelve hours and the sun path sits between its seasonal extremes. That makes it a fair proxy for the shoulder seasons when garden sunlight is valued. Midsummer flatters a scheme and midwinter condemns almost everything, so neither works as a single test.
Does the two-season sunlight test apply to gardens?
No. The annual probable sunlight hours test, which asks for 25 per cent of annual hours including 5 per cent in winter, applies to main living room windows. Gardens are assessed on sun on ground at the March equinox. Citing the winter criterion against a garden is a common mistake in objection letters and weakens the case.
My garden already gets very little sun. Does that help or hurt?
It changes which number matters. If the baseline is already below half, the absolute recommendation cannot be met whatever the applicant does, so the argument shifts to the proportionate loss and the 0.8 ratio. An honest assessment states the failing baseline rather than hiding it, because the officer will notice either way.
Does an existing boundary fence count against the proposal?
No. Shade from an existing fence, garage, hedge or neighbouring outrigger sits in the baseline, so the area it shades is already excluded before the proposal is tested. That is why baseline accuracy decides results, and why modelled heights should come from a survey or a measured drawing rather than from a photograph.
Is overshadowing the same as a right to light?
No. Rights to light is a private law matter concerning easements acquired over particular windows, enforced through the civil courts, and the council does not decide it. Overshadowing and sunlight are planning considerations assessed against guidance. A planning permission gives no protection against a rights to light claim.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.