Sunlight to gardens and the two hour criterion

Sunlight to gardens two hours is the shorthand for a specific test in BRE 209. At least half the area of an amenity space should receive at least two hours of sunlight on 21 March. Where a new development reduces an existing garden below that, and the area still receiving two hours of sun falls below 0.8 times its former value, the guidance treats the loss as noticeable. Both limbs have to be failed before the result counts against a scheme, which is the part objectors and applicants most often get wrong.

It is a ground-level test. It has nothing to do with the sunlight tests applied to windows, which use annual probable sunlight hours and a separate winter criterion. A scheme can pass every window test and fail the garden test, and the reverse happens just as often.

What the sun on ground test measures

The method computes, for every point on a grid across the amenity space, the total number of hours of direct sunlight reaching that point on the assessment date. The output is a plan with a contour separating the area that achieves two hours from the area that does not, usually shaded.

Two details of the method are worth knowing because they surprise people.

The two hours do not have to be continuous. A point that gets 70 minutes in the morning and 55 minutes in the late afternoon, with the proposed building blocking the middle of the day, achieves two hours and counts as compliant. That can produce a technically passing garden that feels overshadowed at the time people actually use it, which is a fair point to make in an objection and a fair point to answer in a design.

And the test measures area, not usability. Half the garden meeting the criterion satisfies the guidance even if the compliant half is the strip along the back fence rather than the terrace outside the back door. Where that happens, say so and show it rather than relying on the headline percentage.

Why 21 March

The spring equinox is used as a representative midpoint. Sun altitudes on 21 March are close to those on 21 September, so a single run covers both shoulder seasons, and the date avoids both the flattering high summer sun and the punishing December sun that would fail almost every urban garden.

Authorities sometimes ask for additional shadow studies on 21 June and 21 December, and sometimes for hourly shadow plots through a single day. Those are useful illustrations and they are not the BRE criterion. Keep them in a separate section of the report labelled as supplementary, so that nobody confuses a June shadow plot with a compliance result.

Which spaces the test applies to

BRE 209 directs the test at spaces where sunlight is expected to matter. In practice that means the main garden of an existing dwelling, communal gardens and amenity areas within a proposed scheme, children's play space, parks and playing fields, and the sitting-out areas of cafes and public houses.

It does not mean every patch of open ground. Front gardens, narrow side passages, parking areas, service yards and access roads are not amenity spaces for this purpose, and including them in the calculation to raise a percentage is the most common manipulation in these reports. Define each assessed area on a plan, with its boundary and its area in square metres, and state why it has been assessed or excluded.

Allotments, orchards and growing space sit in an awkward position, because the two hour criterion was not designed with cultivation in mind and plants have different requirements from people sitting outside. That argument is set out in overshadowing of allotments, orchards and growing space.

Small urban gardens need judgement. A 20 m² courtyard behind a terraced house can fail by a few square metres, and applying the criterion rigidly in that context produces results nobody intended.

Trees and existing structures

Trees are the live methodological argument. Standard practice is to exclude them from the baseline model, on the basis that they can be pruned or removed without consent in most cases, that deciduous canopies admit a good deal of light in March before leaf burst, and that modelling a canopy accurately is guesswork.

Excluding them is defensible, and so is the opposite position where the trees are protected by a Tree Preservation Order or sit within a conservation area and are therefore effectively permanent. What is not defensible is excluding trees without saying so. State the treatment in the methodology and, where a protected tree dominates a garden, run the analysis both ways.

Existing built structures are different. Boundary walls, outbuildings, garages, and the host building itself all have to be modelled, because they are part of the existing condition against which the change is measured. A 2-metre close-boarded fence on the southern boundary of a small garden does real work on a March sun path, and omitting it inflates the existing baseline and therefore exaggerates the loss.

Ground level matters too. State the level the grid sits at and whether it follows site topography, because a sloping garden analysed on a flat plane gives the wrong answer at both ends.

Where a courtyard cannot comply

Some spaces will never pass. A deep courtyard between two five-storey blocks, a north-facing garden backing onto a taller building, an amenity space on the shaded side of a dense perimeter block. Pretending otherwise with a generous area definition helps nobody.

The honest route is to show the result, explain why the geometry produces it, and set out what was done about it:

1. Step or set back the massing on the southern side of the space, which is the only move that changes the fundamental geometry.
2. Relocate the main amenity provision to a roof terrace or a podium where sun access is achievable, keeping the courtyard as circulation and planting.
3. Redistribute the space, providing a smaller but properly sunlit area rather than a large permanently shaded one.
4. Where none of that works, present the shortfall against the benefits of the scheme and ask for a balanced judgement.

That last option is legitimate. BRE 209 is guidance rather than a standard, it says so itself, and it is intended to be applied flexibly in dense urban contexts where rigid application would prevent development that is otherwise acceptable. How local policies pick that up varies considerably, which is covered in local plan amenity policies and how they use BRE 209.

Presenting the plots so an officer can use them

A sun on ground section that works has four parts for each assessed space.

A plan of the space with its boundary and area stated. The existing condition plot, with the two hour contour and the compliant area as a percentage. The proposed condition plot, same scale, same shading convention. And a results table giving existing compliant area, proposed compliant area, the percentage of the space in each case, and the ratio of proposed to existing against the 0.8 figure.

Three presentation habits that save a round of questions. Use the same shading convention and the same orientation on every plan, with a north point on each. Put the assessed boundary on the proposed plot as well as the existing one, so the reader can see nothing moved. And include a short paragraph identifying which part of each garden is compliant, because the officer has to describe the effect in words in the committee report and will otherwise write something imprecise.

Where construction will temporarily worsen the position, through scaffolding, hoardings or a tower crane base in a neighbouring garden, that is a separate matter and should not be mixed into the permanent assessment, as discussed in temporary loss of daylight during construction.

An Overshadowing Study is the document that carries this analysis, and on a scheme that also affects neighbouring windows it should sit alongside rather than inside a Daylight and Sunlight Assessment, with the methodology stated once and applied consistently to both.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect sunlight and overshadowing analysis to be prepared by a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.