Overshadowing of amenity areas and the two hour test

The test is simple to state. At least half of a garden or amenity area should receive at least two hours of sunlight on 21 March. If a new building takes an existing garden below that, and the part still getting two hours falls to less than 0.8 times what it was before, the loss is likely to be noticeable. Those are the recommendations in BRE 209, the guide on site layout planning for daylight and sunlight, and they are what a council's daylight consultant will apply to your garden.

Two hours sounds generous until you plot it. On 21 March the sun is low for much of the morning and afternoon, so a two storey extension on the south side of a boundary casts a long shadow for most of the day.

What the test measures and on which date

It measures direct sunlight reaching the ground, not brightness or daylight. The question is how much of the ground surface of the amenity area receives at least two hours of sun during the day, and the date used is 21 March, the spring equinox.

The equinox is chosen because it sits between the extremes. On 21 June the sun is high and almost everywhere gets some sun, so the test would pass everything. On 21 December the sun is so low that most urban gardens fail, so the test would reject everything. 21 March gives a result that distinguishes between proposals, which is the point of a test.

Some authorities also ask for 21 June and 21 December plots as context, and there is no harm in providing them. The recommendation itself is assessed on the equinox.

The half of the area rule

Two numbers do the work.

The first is the 50 per cent threshold: at least half the amenity area should get two hours or more of sun on 21 March. The second is the 0.8 factor, which applies where the garden fails the first test after development. If the sunlit area has dropped to less than 0.8 times its previous size, in other words a reduction of more than a fifth, the guidance says the loss is likely to be noticeable.

Put together, they mean a garden can end up with less than half of it sunlit and the loss still be judged acceptable, provided the reduction from the existing position is modest. And a garden that stays above half remains compliant whatever the reduction, because the first test is met.

That structure surprises people on both sides. Objectors assume any reduction counts. Applicants assume any garden over half the threshold is unassailable. Neither is right, and the local plan policy wording matters as much as the guidance, which is the subject of local plan amenity policies and how they use BRE 209.

Which amenity areas get assessed

BRE 209 applies the recommendation to gardens and to a list of other open spaces used by people: parks and playing fields, children's playgrounds, outdoor swimming pools and paddling pools, sitting-out areas, and the grounds of hospitals, hotels, schools and similar institutions.

For a householder dispute the relevant area is almost always the main rear garden of the affected house. A few practical points that come up repeatedly:

• The whole garden is normally assessed, not just the part near the house, although some assessors exclude narrow strips and areas behind the building line.
• A front garden used for parking is rarely treated as amenity area.
• Side passages and bin stores are not amenity area.
• A communal garden serving flats is assessed as one area rather than divided between the flats.
• Balconies and roof terraces are not covered by this test and are assessed differently.

Where the space is used for growing rather than sitting, the two hour recommendation understates what is needed, and the thinking on overshadowing of allotments, orchards and growing space is the better starting point.

Gardens that are already overshadowed

This is where the 0.8 factor earns its place. A long thin garden behind a Victorian terrace, with a three storey house to the south and a two storey back addition, may already receive two hours of March sun across only a third of its area. The first test has already failed before anything is built.

In that situation the question becomes proportional. If the proposal reduces the sunlit third to a quarter, that is a reduction of about a quarter and the loss would be judged noticeable. If it reduces it only slightly, the guidance would not flag it.

The practical consequence for a householder is that the existing condition has to be modelled as carefully as the proposed one. An assessment that models the proposed situation accurately and the existing situation crudely will overstate or understate the loss, and that is the first thing to check in a report prepared for the applicant. Ask for both plots at the same scale, on the same base, with the same obstructions shown.

Fences, trees and what counts as an obstruction

Buildings, walls and fences are modelled as they exist. A 1.8 metre close-boarded fence on the south boundary is an obstruction and appears in both the existing and proposed plots.

Trees are different. BRE 209 takes the view that trees and shrubs need not be included, because their shade is seasonal, variable and not permanent in the way a building is. That has an odd result: a garden heavily shaded by a neighbour's sycamore can be assessed as though the tree is not there, and the proposed extension then appears to cause a loss that in practice is masked.

Assessors handle this inconsistently, and officers do too. The honest approach is to model the permanent obstructions in the main assessment, in line with the guidance, and add a note describing the actual conditions including vegetation, so the officer can weigh it. Do not simply leave the trees out and say nothing, and do not model them in without flagging the departure from the guidance.

How the plots are produced and what to ask for

The output is a sun on ground plot, sometimes called a shadow plot: a plan of the garden with the area receiving two or more hours of March sun shaded, usually built from hourly shadow positions through the day.

What a usable report contains:

1. A plan of the amenity area with its boundary marked and its area in square metres.
2. The existing sunlit area and the proposed sunlit area, each as an area and as a percentage of the whole.
3. The ratio of proposed to existing, so the 0.8 factor can be checked.
4. The model's obstruction set, listed, with heights and the source of those heights.
5. A statement of whether vegetation was included.
6. The same information for each affected garden, not an average across several.

Ask for the figures as numbers rather than as coloured drawings alone. A plot that shows two shaded areas without areas or percentages cannot be checked, and a report that cannot be checked carries less weight at appeal. For an objector, the same material is the basis of a submission that officers can actually use, which is the point made in objecting on daylight grounds.

Where the test is the wrong tool

Three situations where the two hour recommendation gives a misleading answer.

Dense city centre sites, where almost no ground level space meets the recommendation and the guidance itself accepts that flexibility is needed. Applying the test rigidly in a city centre would stop all development, and officers know it.

North-facing gardens, which receive very little sun at the equinox in any event, so a proposal can make almost no difference to a result that was always going to fail.

Small courtyards and light wells, where the relevant question is enclosure and outlook rather than hours of sun.

In those cases the argument moves to general amenity, enclosure and overbearing effect, which are planning judgements rather than numerical tests. A numerical Overshadowing Study still helps, because it puts a figure on the change even where the recommendation is not decisive.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

To see which amenity documents a proposal is likely to need, enter your postcode on PlanScope and answer the guided questions. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.