Sunlight to gardens and amenity areas on 21 March

A sunlight to gardens assessment asks one question: on 21 March, does at least half of the garden or amenity area get at least two hours of direct sunlight? That is the test in the BRE guidance on site layout planning for daylight and sunlight, and it is what a case officer is reaching for when a neighbour objects that a new extension or block will put their garden in shadow. There is a second limb. Where a garden already meets the test and a new building takes it below, the loss is treated as noticeable if the sunlit area drops to less than 0.8 times what it was.

This is a different exercise from sunlight to windows, which uses annual probable sunlight hours across the whole year. Gardens are assessed on the ground, on one date.

Why the equinox date is used

21 March is the spring equinox. The sun follows a path midway between its highest summer arc and its lowest winter one, day and night are close to equal, and the result sits in the middle of the range rather than at either extreme.

Test in June and almost everything passes, because the sun is high and clears obstructions easily. Test in December and almost nothing does, because a low sun throws shadows several times the height of the building casting them. The equinox produces a result that discriminates between schemes, which is the point of a test.

It also has the practical merit of being reproducible. Two consultants modelling the same geometry on 21 March should produce the same sunlit area, and where they do not, the difference is in the model rather than the method.

The two hour criterion and the half area test

The criterion combines an amount of sunlight with a proportion of the space. Two hours of direct sun on 21 March, over at least half the area.

Two hours does not have to be continuous. A garden that gets an hour in the morning before the neighbour's house intervenes and another hour in the afternoon once the sun clears it meets the criterion. The modelling sums the sunlit periods for each point on the ground.

The half area figure is a proportion of the amenity space being assessed, so how you draw the boundary of that space changes the answer. A long thin back garden where the near third is permanently shaded by the host dwelling may still pass because the far two thirds are open. Move the assessment boundary to exclude a shed and a paved yard and the percentage shifts.

The 0.8 comparison handles existing gardens. It matters because a garden can be well above the threshold before development and still lose a lot of sunlight.

Which spaces count as amenity areas

The guidance is not limited to private back gardens. It names gardens to existing and proposed dwellings, parks and playing fields, children's playgrounds, outdoor swimming and paddling pools, and sitting-out areas at places such as hospitals and care homes.

What is generally not assessed: driveways, bin stores, access paths, front gardens used for parking, and land with no amenity function. An objector who argues that their side passage has lost sunlight is on weak ground.

Balconies and roof terraces sit awkwardly. They are amenity space, they are often the only outdoor space a flat has, and the guidance was written with ground-level areas in mind. Assess them, say what method you used, and be clear that the criterion is being applied by analogy. Shared courtyards in flatted schemes have their own difficulties, which we cover in courtyard blocks and sunlight to shared amenity space.

Existing gardens against proposed communal space

The two cases behave differently and applicants often run them together.

For an existing neighbouring garden, the analysis is a before and after comparison and the baseline is whatever is actually there: the neighbour's own house, their garage, the mature conifer hedge, the boundary fence. All of it goes in the model. Leaving out an existing obstruction that already shades the garden overstates the effect of your proposal, and an objector's consultant will find it.

For proposed communal space in a new development, there is no before. The criterion is applied to the design as drawn, and the honest reporting of a courtyard that achieves 38 per cent rather than 50 per cent is a statement about the layout, to be weighed against density, aspect and the other benefits. Do not fudge it. Officers read these numbers and committee members quote them.

Trees are the recurring argument. A deciduous tree on 21 March is largely bare, so modelling it as a solid block overstates shading, while modelling it as absent understates the reality in summer. State the assumption you made either way.

Boundary fences, levels and sloping sites

Boundary treatment changes the result at ground level more than people expect. A 1.8 m close-boarded fence on the south side of a small garden casts a shadow several metres long at the equinox, and on a narrow plot that alone can take the sunlit area below half.

Levels matter as much. A garden 1 m below the proposed building is effectively facing a building 1 m taller. On a site with a fall across it, model the actual ground surface rather than a flat plane, because a flat plane assumption on sloping ground can be wrong in either direction.

Two further details worth getting right: the ground surface in the model should be the finished level after any regrading, and retaining walls and raised planters count as obstructions.

Presenting the sun on ground plan

The output that persuades is a plan of the amenity area with the two hour sunlit zone shaded, shown for the existing and proposed cases side by side, with the percentage stated on each.

Include the modelled geometry so the assumptions are visible: building heights used, whether they were surveyed or taken from drawings, how trees and fences were treated, and the assessment boundary for each space. A single table of percentages with no plan is what gets challenged.

Where a garden fails, say what would fix it and what it would cost the scheme. A reduction in depth at one corner, a hipped roof instead of a gable, a set-back at second floor. Presenting a tested alternative is far stronger than asserting that no alternative exists. For the wider question of how to lay results out for a committee audience, see rear extensions and overshadowing in dense terraces.

An illustrative example, to show the shape of the numbers rather than to be relied on: a 3 m deep single-storey rear extension on the south side of a pair of semis, with a 45 m² neighbouring garden that currently receives two hours of March sun across about 70 per cent of its area. Extending at single storey with a flat roof takes it to around 60 per cent, comfortably above half and above 0.8 of the former value. Taking the same extension to two storeys takes it to roughly 45 per cent, which fails both limbs. The variable that moved the result is height, not depth, which is usually the case.

What this does and does not decide

Passing the test does not guarantee that a scheme is acceptable on amenity grounds, and failing it does not make refusal inevitable. The BRE guidance describes itself as guidance rather than a standard, and it says its numerical criteria may be applied flexibly where circumstances warrant. Some local plans set their own amenity standards and a few boroughs apply stricter figures for communal space, so read the adopted policy alongside the guidance.

Councils expect a suitably qualified author for daylight and sunlight work, and professional review of any draft is strongly advised. If you want the scope set out before you commission anything, the Overshadowing Study and Daylight and Sunlight Assessment pages describe what each covers, or enter your postcode on PlanScope to see which reports your site is likely to need.

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.