Sun on ground: the two hour test for amenity areas

The sun on ground amenity test asks whether at least half of an amenity area receives at least two hours of direct sunlight on 21 March. That is the guideline in BRE 209, and it is the figure a case officer will look for when a neighbour says the new building will put their garden in shadow.

The second part of the guideline is the one people forget. Where an existing space no longer meets the two hour target after development, the loss is treated as noticeable if the area receiving two hours of sun falls below 0.8 times its former value. So a garden that already performed badly is judged on the relative change as well as on the absolute result.

What the test measures

It measures direct sunlight reaching ground level, not sky visibility and not illuminance. The output is a plan of the amenity area showing which parts receive two or more hours of direct sun on the assessment date, expressed as a percentage of the area.

Two runs are needed for an existing space: the condition before the proposed development and the condition after it. For a new space within a proposed scheme, only the proposed condition applies, with the scheme's own massing and any surrounding buildings modelled.

An Overshadowing Study usually presents both the sun on ground result and a set of transient shadow diagrams through the day. The two answer different questions, and the difference between them is explained in our note on transient overshadowing and permanent loss.

Why 21 March is the assessment date

The equinox is the midpoint of the year for solar geometry. Sun paths on 21 March sit between the summer extreme and the winter extreme, so a result on that date describes typical conditions rather than a best or worst case.

Assessing in June flatters every scheme, because the sun is high and shadows are short. Assessing in December condemns almost every urban space. Neither tells a decision maker anything useful about the space in ordinary use.

Some reports add 21 June and 21 December plots as context, and that is helpful where a space is intended for summer use or where the scheme sits in a deep urban block. Add them as supporting material. The assessment itself stays on 21 March, and a report that quietly moves the assessment date to a more comfortable one will be picked apart.

The proportion of the area that must receive sun

The guideline is that at least half of the amenity area receives at least two hours of sun on the assessment date. Report the figure as a percentage and state the area in square metres alongside it.

The third row is the one that generates argument. A space that was already poor and gets marginally worse passes the relative test while failing the absolute one, and the honest way to report it is to give both figures and say which the scheme relies on.

Dense urban sites frequently cannot meet the guideline, and BRE 209 says its numerical guidance should be applied flexibly according to the context. That is not a licence to ignore the result. It is an invitation to explain why the result is acceptable here, with reference to what else the scheme provides.

Which spaces count as amenity areas

BRE directs the assessment at spaces where sunlight matters to the use: the main back garden of a house, the main amenity space serving a block of flats, parks, playing fields, children's playgrounds and sitting out areas.

For an existing dwelling, assess the principal garden rather than every strip of land in the title. A side passage, a bin area and a narrow north facing strip are not the amenity space, and including them dilutes the result in both directions.

For a proposed scheme, assess the spaces the residents will actually use: the communal garden, the podium, the play space, the roof terrace if it is the main provision. Where play space is provided, sunlight matters more rather than less, and a similar logic applies to school playgrounds and nursery outdoor space.

Private balconies are not usually assessed as amenity areas under this test, though they are relevant to the wider amenity judgement and to overheating.

Modelling surrounding obstructions

The model has to include everything that casts a shadow on the space at the times assessed, and that means more than the proposed building alone.

That means existing buildings around the site at their real heights, boundary walls and fences, and the proposed scheme itself. Ground level changes matter on a sloping site, because a shadow cast onto rising ground reaches less far than the same shadow on the flat.

Trees are the judgement call. Deciduous trees are largely bare on 21 March, so the convention is to exclude them or to model them as a light filter and say which was done. Evergreen trees and dense conifer screens should be included, because they are opaque all year.

State the survey source for the surrounding context. A model built from an aerial survey with assumed storey heights is acceptable for early testing and weak as submitted evidence. On a contested scheme, measured heights are worth the cost.

Courtyards and podium gardens

Perimeter blocks concentrate the problem. A courtyard surrounded on four sides will meet the guideline only if it is wide relative to the surrounding heights, and the useful sunlit area usually sits along the northern side of the space rather than in the middle.

Three moves help more than anything else. Open the southern edge of the courtyard, either by dropping the block height there or by breaking the perimeter. Set the podium level above the shadow of the surrounding street wall where the section allows. And place the seating, the play equipment and the planting in the part of the space that the study shows receives sun, rather than in the geometric centre.

That last point is design rather than assessment, and it is the one that turns a marginal number into a space people use. A courtyard at 45 per cent with the sunlit part landscaped as the main sitting area is a better outcome than 55 per cent arranged without reference to the study.

What to do when the test fails

Run the assessment early, while the massing can still move. A Daylight and Sunlight Assessment commissioned after the elevations are resolved can only report the problem.

Where the scheme still falls short, the options are limited and worth stating plainly in the report. Reduce height or set back the southern element. Relocate the amenity space to a sunnier part of the site. Increase the quantum of amenity space so that the absolute sunlit area is larger even though the percentage is not. Or accept the shortfall and make the planning balance argument, supported by what the scheme delivers elsewhere.

Do not present a failure as a pass by changing the boundary of the assessed area. Officers compare the assessed extent against the landscape drawing, and an amenity area that shrinks between the two documents is the fastest way to lose credibility on everything else in the report.

Browse all 66 report types to see how sunlight evidence sits alongside the rest of a submission.

This is general guidance rather than 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.