Overshadowing studies: what an officer expects to see

An overshadowing study is a set of shadow drawings and area calculations that answers one question: how much less sun will the neighbouring garden, window or amenity space get once the proposal is built. Officers ask for it when a scheme is close to a boundary, taller than what it replaces, or already attracting objections about light. What they expect to see is narrower than most people assume, and the studies that get sent back usually fail on the model rather than on the result.

If you are building a new house in a back garden or adding a second storey near a boundary, commission the study before the design is fixed. It costs less to move a wall on a drawing than to defend a shadow at committee.

Agreeing the scope before anyone models anything

Three decisions define the study, and getting them agreed with the council at pre-application saves a round of revisions.

Which receptors. Normally the gardens and principal windows of the properties that adjoin the site, plus any amenity space the scheme itself creates. On a corner plot or a stepped terrace, the affected properties are not always the obvious two.

Which dates. The spring equinox on 21 March is the primary test date for sunlight to gardens. Many councils also ask for 21 June and 21 December to show the summer and winter extremes. Some ask for equinox plus two intermediate dates.

Which cases. At minimum, existing and proposed. Where a neighbouring scheme already has permission, a cumulative case showing both is worth including, because leaving it out invites the objection that the study understates the position.

What has to be in the model

This is where studies fail. A shadow calculation is only as good as the geometry behind it.

• A topographic survey with levels, because a site that falls away from the boundary produces a different shadow from a flat one.
• The existing building being retained or replaced, modelled accurately rather than as a box.
• Neighbouring buildings, at their real heights and positions, including rear extensions and outbuildings.
• Boundary walls and fences at their actual heights. A 1.8 m close-boarded fence already shades a strip of garden and it belongs in both the existing and proposed cases.
• Retained trees, where they are protected or certain to remain. Trees that may be removed should be shown in a sensitivity case rather than quietly deleted.
• The correct north point, taken from the survey rather than from a site plan of uncertain origin.

State the coordinate basis and the source of every element. An officer who cannot tell where the neighbour's ridge height came from will not rely on the output.

Times, and the detail almost everyone gets wrong

Shadow plots are calculated in solar time, which is not the same as the time on a clock in Britain in summer. Whether the drawings are labelled in solar time, Greenwich Mean Time or British Summer Time changes what the reader thinks they are looking at.

Pick one convention, state it on every drawing, and use it consistently. Hourly intervals through the day are the norm, with half hourly steps where the shadow moves quickly across a small garden.

The outputs officers actually read

A study that hands over forty images without a conclusion is harder to use than one with six drawings and a table.

1. Shadow plans at hourly intervals on each test date, existing and proposed, drawn at the same scale with the same rendering.
2. Sun hours on ground plots, shading the area that receives two hours or more of sunlight on 21 March in each case.
3. A table giving the sunlit area of each assessed garden, in square metres and as a percentage of the total, for the existing and proposed cases.
4. The ratio of proposed to existing sunlit area, since a fall below 0.8 is treated as a noticeable loss under the BRE guidance.
5. A short written conclusion for each receptor, stating the numbers rather than describing the images.

Include a plan showing which areas were assessed and which were excluded, with the reasons. Excluding a side passage is reasonable. Excluding the half of the garden nearest the extension is not, and it will be spotted.

Where the study fits in the wider assessment

Overshadowing is one part of a family of tests, and mixing them up muddles the argument.

An Overshadowing Study addresses the first row and is usually enough for a householder or small residential scheme. Where habitable room windows are affected as well, the fuller Daylight and Sunlight Assessment adds the window calculations and is what larger schemes need.

Solar panels on a neighbouring roof are worth a sentence. Loss of output from an existing installation is capable of being a material consideration, and where the affected roof carries panels, model them and address the point rather than waiting for the objection.

The failings officers find

Five recur.

The existing case includes something that is not there, usually a tall hedge or an outbuilding the applicant intends to remove. The proposed case omits something that is, usually the boundary fence.

Neighbouring buildings are modelled as flat-roofed blocks at eaves height, which shortens the existing shadow and makes the proposal look worse than it is, or as ridge-height blocks, which does the reverse.

The assessment date is labelled 21 March but the sun position corresponds to a different latitude, because the model was set up with a default location.

The conclusion asserts that the impact is acceptable without giving the areas or the ratio.

And the study assesses the wrong garden. On terraced streets the property most affected is often diagonally behind rather than directly adjoining.

Illustrative example. A two storey side and rear extension on a semi-detached house shows a proposed sunlit garden area at 0.76 of the existing figure, below the 0.8 threshold. Reducing the rear projection by 600 mm and dropping the eaves on the boundary side brings the ratio back above 0.8. The revised study is two drawings and a table. This example is illustrative only.

Submitting it

For a householder or minor application, the study goes in as a standalone document with the drawings appended, and its conclusions are summarised in the Design and Access Statement where one is submitted. For anything larger, the amenity conclusions belong in the Planning Statement alongside the other material considerations.

Where a neighbour has already objected, send the study to the case officer as soon as it exists rather than holding it for a formal amendment. Officers write their reports over a short window, and evidence that arrives after the draft is written carries less weight than evidence that arrives before.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that the BRE numerical values are guidance to be applied with judgement rather than fixed limits.

See PlanScope pricing if you want to draft the supporting statements while a specialist runs the modelling.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.