The two hour sunlight test for gardens and amenity space

The test most English councils apply to garden sunlight comes from the BRE guide to site layout planning for daylight and sunlight, and it is easier to state than most planning rules. At least half of a garden or amenity area should receive at least two hours of sunlight on 21 March. If a new building reduces the sunlit area to less than 0.8 times what it was, the loss is treated as noticeable.

That is the whole test. Half the garden, two hours, the spring equinox, and a 0.8 factor for the change.

It is guidance rather than law, and the BRE guide says explicitly that its numerical values should be applied with flexibility. Councils still use it, because it is the only widely accepted yardstick for something neighbours argue about constantly.

Why 21 March and why two hours

21 March is the spring equinox, when day length is roughly equal everywhere and the sun sits midway between its winter and summer paths. Testing on that date avoids two distortions: a June test would make almost any garden pass, and a December test would fail almost all of them.

Two hours is a judgement about usable outdoor amenity rather than a scientific threshold. A garden that gets two hours of March sun will get considerably more in high summer, which is when people use it.

The area assessed is the part of the garden in ordinary use. For a house that usually means the whole rear garden, sometimes excluding a narrow side passage or a shed footprint. For a block of flats it means the communal amenity area rather than the sum of every scrap of landscaping. Where a garden is already divided by dense planting or a level change, say which area you have assessed and why.

The 0.8 factor, and what it actually measures

The two hour test looks at the situation after development. The 0.8 factor compares before with after.

Take the area of garden receiving two hours of sun on 21 March in the existing situation. Then take the same area in the proposed situation. If the proposed area is less than 80 per cent of the existing area, the reduction is treated as noticeable, and the council has to weigh it.

Both parts matter, and they can pull in different directions:

That last row is the one householders find counter-intuitive. A north-facing garden hemmed in by mature trees may already fail the two hour test with nothing built. The proposal is then judged on how much it makes an existing shortfall worse, rather than on the absolute number.

What the assessment looks like

Two outputs do the work.

Sun on ground plots show the garden in plan with the sunlit area shaded at intervals through 21 March, and a summary of the total area receiving two hours or more. Produce them for the existing and proposed situations so the 0.8 comparison can be read directly.

Transient overshadowing diagrams show the shadow cast at hourly intervals, usually on 21 March, with 21 June and 21 December added where the council asks. They are easy to read and easy to check, which is why officers like them.

Both come out of a three dimensional model that has to include the existing building, the proposal, neighbouring buildings, boundary walls and fences of the right height, and any trees that are protected or that will certainly remain. Leaving out a two metre close-boarded fence is a common way to produce an answer that does not survive scrutiny.

An Overshadowing Study covers this ground for a householder or small scheme. Where windows as well as gardens are affected, the wider Daylight and Sunlight Assessment adds vertical sky component, no-sky line and annual probable sunlight hours calculations for the affected rooms.

Where garden sunlight sits among the other amenity tests

Loss of light to a garden is a material planning consideration, and so is loss of light to habitable room windows. They are assessed differently.

Windows are tested using the vertical sky component, with 27 per cent as the reference value, and the no-sky line inside the room. Sunlight to windows uses annual probable sunlight hours, applied to windows within 90 degrees of due south. Gardens use the two hour equinox test described here. A scheme can pass the window tests and fail the garden test, or the reverse.

Many councils also operate a 45 degree or 25 degree rule in a householder design guide, drawn in plan and in section from the centre of the neighbour's nearest window. Those rules are a quick proxy that sits alongside the BRE method rather than replacing it. Where a scheme fails the proxy but passes the BRE test, say so with the modelling to back it.

Rights of light is a separate matter entirely. It is a private legal right that can arise over time, enforced through the civil courts, and the council does not deal with it. A planning permission does not authorise an interference with a right of light, and a right of light claim does not stop a planning permission.

Designing to pass rather than arguing afterwards

The variables that move the result are the ones you can control at sketch stage.

• Depth of the extension along the boundary, which drives the length of the shadow into the neighbour's garden.
• Height at the boundary, particularly the eaves height and whether the roof slopes away.
• Orientation. The same extension on a south-facing rear elevation and a north-facing one produces very different shadows.
• Distance from the boundary, where even 500 mm can change the outcome on a narrow plot.
• Roof form, since a hipped or pitched roof sloping away from the boundary casts a shorter shadow than a flat roof of the same ridge height.

Illustrative example. A single storey rear extension 4 m deep on a terrace with west-facing gardens casts a long afternoon shadow across the neighbour's patio in March. Pulling the extension 700 mm off the boundary and hipping the roof away restores enough of the sunlit area to pass the 0.8 comparison, at the cost of a small amount of internal floor space. This example is illustrative only.

If a neighbour has already objected on light grounds, commission the modelling before the council determines the application rather than after. An officer who has an assessment in front of them can address the objection in the report. An officer with nothing has to take the objection at face value.

What to submit and when

For a householder application, the modelling normally goes in as a short appendix to the Design and Access Statement or as a standalone note with the plots attached. For a larger scheme it is a standalone assessment.

Check the council's own supplementary planning document first. Some set their own garden amenity standards, including minimum garden areas and separation distances, and those sit alongside the BRE test rather than instead of it. Where the local guide and the BRE guide disagree, address both.

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

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