Overshadowing a neighbouring garden: the assessment

An overshadowing assessment of a neighbouring garden is a shadow study: a three-dimensional model of your proposal and the surrounding buildings, run against the sun path, producing plans that show where the shadow falls at stated times on stated dates. For a householder extension the dates that matter are 21 March, with 21 June and 21 December as context, and the test that counts is whether at least half of the neighbour's garden still receives two hours of direct sun on 21 March. Overshadowing of a neighbour's garden is assessed against the amenity space guidance in BRE 209, not against the daylight tests used for windows.

If your extension is a single storey rear addition on a south-facing boundary, the study will usually help you rather than the objector.

What an overshadowing study shows

It shows shadow, at a moment and cumulatively, and nothing else. It does not show loss of light to a room, it does not show loss of privacy, and it does not show loss of a view.

The outputs are a set of shadow plots and a sunlight availability plan. Shadow plots are snapshots: the shadow outline at, say, each hour from 09:00 to 16:00. The sunlight availability plan aggregates them into a map of how many hours each part of the garden receives, which is the drawing that answers the question.

An Overshadowing Study is a focused document. Where windows are also affected, the broader Daylight and Sunlight Assessment covers vertical sky component, no sky line and annual probable sunlight hours, and those are different calculations with different thresholds.

Officers read the aggregate plan. Objectors quote the single worst snapshot. Both drawings should be in the submission so the comparison is visible.

Shadow plots at the equinox and the solstices

Three dates, three purposes.

21 March, the spring equinox, is the assessment date in the guidance, because the sun altitude at the equinox is the midpoint of the annual range and the result correlates reasonably with usable amenity across the year. This is the date the half-area and two-hour test is applied to.

21 June shows the best case. It is useful for showing that the garden is well used in summer, and it is not evidence that the scheme passes, because almost everything passes in June.

21 December shows the worst case. In a British winter the sun is so low that a two-storey building casts a shadow several times its height, and a December plot often shows an entire garden in shade whether or not your extension exists. That makes it useful for exactly one argument: showing that the garden was already shaded in winter by the existing house, so the proposal changes little.

Model the existing situation on each date as well as the proposed. The comparison is the evidence.

Transient shadow against total hours of sun

A shadow that crosses a garden for forty minutes in the morning is not a loss of amenity. A shadow that sits on the patio from noon onwards is.

This is the distinction that resolves most neighbour disputes, and it is why single snapshots mislead. The sun moves roughly 15 degrees an hour, so a shadow sweeps. The relevant question is how long any given part of the space is in shade, and whether the parts that lose sun are the parts that were used.

A useful additional output is a shadow plot at the time the neighbour actually uses the garden, if that is known from their objection. If somebody says they lose their afternoon sun, model mid-afternoon specifically and show what happens. Where the claim is right, the honest response is a design change rather than a cleverer drawing.

Illustrative example: a 3m deep single storey rear extension on a 1930s semi, on the north side of the boundary with the neighbour to the south. The shadow falls back onto the applicant's own garden for most of the day. The study takes twenty minutes to produce and ends the objection.

Fences, outbuildings and existing shade

Model the existing boundary treatment and anything already standing, because the baseline is what the proposal is judged against.

A 1.8m close-boarded fence on a southern boundary already casts a band of shade several metres deep at the equinox. A mature garage, a large shed, an existing two-storey outrigger and the neighbour's own extension all contribute. If the model omits them, the proposed-case shadow appears to be entirely yours, and the existing-case comparison that would have helped you has been thrown away.

Trees are treated differently. Deciduous canopy is not in leaf on 21 March and shade from trees is generally regarded as a benefit in amenity space rather than a harm, so vegetation is usually excluded from shadow models. Dense evergreen screens are worth describing in the text, and where the scheme removes trees, the existing case should not claim credit for shade that is about to go.

Levels matter more than people expect. A garden that falls away to the south is shaded less than a flat one; a garden rising to the south is shaded more. Model the ground, not a flat plane.

When a council asks for the study

Rarely for a modest single-storey extension, and routinely in four situations.

Where the proposal is two storeys or more on a boundary with a garden to its north. Where a local plan policy or a householder design guide sets a specific overshadowing or sunlight test. Where objections have raised overshadowing and the officer needs something to weigh. And where the proposal is a new dwelling in a back garden, because the new garden and the retained one both have to work.

Some councils set their own rules of thumb, commonly a 45-degree test in plan and section taken from the nearest neighbouring window or boundary, and a scheme that passes a 45-degree test often does not need a shadow study at all. Check the council's householder design guidance before commissioning anything, because a drawn 45-degree test may be all that is wanted.

Design responses that reduce overshadowing

Four moves do most of the work, and they are cheaper than argument.

Pull the extension off the shared boundary. Even 500mm of separation reduces the shadow band on the neighbour's side noticeably at low sun angles. Reduce height at the boundary: a hipped or mono-pitched roof falling towards the neighbour removes the top of the shadow, which is the part that reaches furthest. Shorten the depth of the element nearest the boundary and keep the deeper part away from it, stepping the plan. And set the building lower where levels allow, by reducing the floor-to-ceiling height or dropping the slab, since shadow length is proportional to height.

Glazing does not reduce overshadowing. A glass roof casts very nearly the same shadow as a tiled one. It is a useful daylight move inside the extension and irrelevant outside it.

What the study cannot resolve

A shadow study answers a factual question and leaves three others open.

It does not determine the planning decision. The numbers from BRE 209 are guidance, and the officer weighs them against the character of the area, the existing quality of the space and whether a reasonable alternative design exists. A scheme that technically fails can be permitted, and one that passes can be refused on other amenity grounds.

It has nothing to say about privacy or overlooking, which is a separate assessment about sightlines and separation distances.

And it has nothing to do with rights of light, which is a civil law matter about an acquired right to light through a defined aperture, decided between the parties and enforceable in court regardless of what the council decides. The difference is set out in rights of light contrasted with planning amenity conditions, and how officers handle the objections themselves is covered in neighbour daylight objections and how officers weigh them.

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.