The two hours of sun test for gardens on 21 March
At least half a garden should receive two hours of sun on 21 March. Where it already receives less, the test becomes whether the sunlit area falls below 0.8 times its former size. Here is how both are assessed.
The garden test in the BRE guidance asks whether at least half of an amenity area receives at least two hours of sunlight on 21 March. If it does before your extension and does not afterwards, expect an objection. If less than half already receives two hours of sun, the further question is whether the area that does is reduced to less than 0.8 times its former size, which is the point at which the guidance treats the loss as noticeable.
Two hours on 21 March is a modest standard. It exists because the spring equinox gives an average day length and an even sun path, so a result on that date stands for the year rather than flattering the scheme.
What the test asks and which spaces it covers
The test applies to gardens and amenity areas, meaning the outdoor space people use for sitting, eating and drying washing. For a house that is normally the principal rear garden. Front gardens, narrow side passages and areas given over to parking are generally excluded, because nobody sits in them.
For flats it applies to the communal amenity space and to any private garden. Balconies are usually assessed differently or not at all, since the test is designed for areas of ground. The guidance also extends to open spaces used by the public, play areas and allotments, where the same two-hour measure is applied to the space as a whole.
The area assessed is the useable garden, not the title boundary. A garden with a large outbuilding at the end is assessed on what is left.
The half-area threshold and the assessment date
Two figures do the work. Half the area, and two hours of direct sun on 21 March.
| Position before development | Test applied |
|---|---|
| At least half the area receives two hours of sun | Retain at least half the area receiving two hours |
| Less than half already receives two hours | Area receiving two hours should not fall below 0.8 times its former value |
The date matters. Assessing on 21 June makes almost any garden pass, and assessing on 21 December makes almost any garden fail. Some reports present 21 June and 21 December alongside the equinox to show the range across the year, which is useful context provided the conclusion is drawn from 21 March.
The measure is cumulative sunlight over the day rather than two unbroken hours. A garden that gets an hour in the morning and an hour in the late afternoon passes, even though the middle of the day is shaded.
Modelling assumptions that change the answer
The result depends on what goes into the model, so the assumptions belong in the report.
- Boundary treatment at its existing height, with the height stated. A 1.8 metre fence casts a real shadow on a small garden in March.
- Existing buildings, including outbuildings and neighbouring extensions, surveyed rather than estimated.
- Ground level changes, because a garden that rises away from the house behaves differently.
- Deciduous trees, which are normally excluded on 21 March because they are not in leaf, with a note where an evergreen hedge is the shading element.
- The assessment grid spacing, and whether the area is measured as a percentage or in square metres.
Trees cause more arguments than anything else. A neighbour's leyland cypress hedge may be the reason a garden is shaded, and it is not a material consideration against your extension. Model the built form, note the hedge, and say which one is responsible for the result.
Existing overshadowing before development
The baseline is where householder cases are won. Many small urban gardens already fail the two-hour test because of boundary fences, mature planting and the neighbouring terrace, and where that is the position the test shifts to the 0.8 comparison.
Run the baseline first and report it honestly. A report that shows 38 per cent of the garden receiving two hours before development and 32 per cent after is a stronger document than one that only shows the after case, because the officer can see the proportionate effect. And where the baseline is poor, an extension that removes very little sunlight is easier to defend than a scheme presented against an idealised starting point.
Our note on what a case officer checks in a daylight report first covers the order in which these results are read.
Balconies, communal spaces and shared gardens
On a block of flats the communal garden is treated as one amenity area, which produces a different result from testing each private garden separately. Where the communal space is the only amenity provision, officers look at it closely, particularly on infill schemes that build over part of it.
Private balconies and terraces are usually left out of the calculation. That does not mean they are ignored, since a balcony reduced to permanent shade is an amenity point an officer can raise under a general amenity policy without reference to the BRE measure.
Where a scheme provides new amenity space, the same test is the sensible way to check it during design rather than after submission. An Overshadowing Study is the document that presents sun-on-ground results for existing and proposed amenity areas.
Allotments, play space and public open space
The two-hour measure is applied to allotments and play space as a whole rather than plot by plot. For allotments the growing season matters more than the equinox, so a study that also shows the position in May and July is more useful to the officer, even though the guidance threshold is set on 21 March.
Play space raises a separate question about summer shade. A play area with no shade at all in July is a legitimate design criticism, and it is the opposite of the sunlight concern, so a scheme sometimes has to answer both.
What the council does with the result
Officers use the test as one amenity consideration among several, alongside overlooking, the effect on windows and the character of the area. A marginal failure on the garden test rarely refuses an application on its own. A clear failure combined with a loss of daylight to a habitable window usually does.
That interaction is worth understanding before the design is fixed. Shortening a rear extension by half a metre often moves both results at once, because the shadow at 9am and the angle to the neighbour's window are governed by the same depth.
Presenting a shadow study with the result
The presentation that works is short. A plan of the amenity area with its extent shown and its area stated, sun-on-ground plots at hourly intervals on 21 March for the existing and proposed cases, and a single table giving the area receiving two hours before and after with the percentage and the ratio.
Include the failures. A report that shows a 0.72 ratio and explains why the remaining garden still functions, with the depth of the extension reduced by 300 mm as mitigation, reads as honest work. A report that quietly redefines the amenity area to exclude the shaded strip does not, and officers notice.
The full Daylight and Sunlight Assessment covers windows as well as gardens, and for a rear extension the window test usually runs alongside the garden test. Our note on the 25 degree test in section for a rear extension sets out that side of it.
This is general guidance, not legal advice. The BRE measures are guidance rather than policy, and councils apply them alongside their own amenity policies. 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
Why is 21 March used for the sunlight test?
The spring equinox gives an average day length and a sun path midway between the summer and winter extremes, so a result on that date stands for the year rather than flattering the scheme. Testing on 21 June would let almost any garden pass, and testing on 21 December would fail almost any garden.
Does the two hours have to be continuous?
No. The measure is cumulative direct sunlight across the day, so a garden receiving an hour in the morning and an hour in the late afternoon meets the two-hour figure even if the middle of the day is shaded. What matters is the total duration on each part of the amenity area.
Which parts of a garden are assessed?
The useable amenity area, meaning the space people sit in, eat in and dry washing in, rather than the whole title boundary. Front gardens, narrow side passages and parking areas are generally excluded, as is the footprint of a large outbuilding. State the extent you assessed on a plan with its area.
What if my neighbour's garden is already shaded by trees?
Deciduous trees are normally excluded from a 21 March assessment because they are not in leaf, and an existing hedge or tree is not a material consideration against your extension in any case. Model the built form, note the planting, and say plainly which element causes the shading in the baseline.
What is the 0.8 ratio for?
It applies where less than half the amenity area already receives two hours of sunlight. In that situation the guidance compares the sunlit area after development with the area before, and treats a reduction to less than 0.8 times the former value as a noticeable loss. Report the ratio rather than only the percentage.
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.