Sunlight to gardens in a back-land terrace scheme

Back land development and garden sunlight is the issue that decides most rear garden infill applications, and the arithmetic is unforgiving. A two storey house on a back land plot south of an existing terrace will cast a shadow across most of a standard terrace garden at the spring equinox. If your plot sits north of the gardens it affects, the same house causes almost no loss at all. Orientation does more work here than design quality, and the sooner you establish which situation you are in, the less money you spend finding out.

Why back land sites concentrate the problem

A back land plot is surrounded on three or four sides by other people's gardens, and terrace gardens are narrow. That combination produces two features you do not get on a street frontage site.

The affected gardens are small, so a shadow that would be a minor loss on a half-acre plot removes a large proportion of a 5 metre wide garden. And the number of affected parties is high: a single infill dwelling can have six or eight immediate neighbours rather than two.

There is a planning status point as well. Residential gardens in built-up areas are excluded from the definition of previously developed land in the Framework glossary, so a back land scheme does not get the benefit of being brownfield. Where an authority has a policy resisting the loss of garden land, sunlight evidence becomes the proof that the scheme does not cause the harm the policy is aimed at.

The geometry: shadow length at the equinox

The relevant test for gardens is sun on ground, assessed at the spring equinox on 21 March. The sun's altitude at solar noon on that date is roughly 90 degrees minus your latitude, which is where the shadow lengths come from.

Take an illustrative calculation. In southern England the noon altitude at the equinox is around 38 degrees, so a vertical obstruction casts a noon shadow of about 1.25 times its height. In the north of England the altitude is nearer 36 degrees and the factor is about 1.35. Away from noon the sun is lower and shadows are considerably longer.

These figures are illustrative and derived from the geometry above. A typical Victorian terrace garden is between 8 and 15 metres deep, which is why a two storey back land house to the south of those gardens is so difficult, and why a single storey one often is not.

Run this calculation on a napkin before you commission anything. If the numbers are hopeless, the design will not rescue them.

Testing existing and proposed conditions

The BRE guidance on site layout planning for daylight and sunlight sets the test that councils apply. For an amenity area, at least half of it should receive at least two hours of sunlight on 21 March, and where the proportion falls below that and is reduced to less than 0.8 times its former value, the loss is likely to be noticeable.

Two features of that wording matter to a small developer.

It is a comparative test as well as an absolute one. A garden that already receives two hours of sunlight on only 40 per cent of its area is not required to be improved, and a modest further reduction may still pass the ratio. So the existing condition is often your strongest evidence.

It is measured over area, not at a point. Losing sunlight from the end of the garden nearest the house, which is where people sit, can matter more in planning terms than the percentage suggests, and an officer may say so even where the figures pass. Address it rather than relying on the number.

Produce the analysis as an Overshadowing Study with sun on ground plots for the existing and proposed conditions at 21 March, plus 21 June and 21 December as context. Where neighbouring windows are also affected, a Daylight and Sunlight Assessment covers those with the vertical sky component and sunlight hour tests, and the two documents should share one drawing set.

What the boundary wall already does

This is the part applicants consistently fail to use, and it is often the difference between an approval and a refusal.

Terrace gardens are enclosed by walls and fences, commonly 1.8 to 2 metres high, and on the southern boundary of a narrow garden those already remove sunlight from a strip along the base of the wall. Garages, sheds, outriggers, conservatories and mature boundary trees do the same.

Model all of it in the existing condition. A study that models a flat, empty garden as the baseline and then introduces your house produces an artificially large loss, and an objector's consultant will produce the corrected version. Survey the boundary heights rather than assuming them, and include the neighbours' own extensions, because a rear outrigger on a Victorian terrace casts a long morning or afternoon shadow across its own garden.

The same logic applies in reverse where the receiving space is used for growing rather than sitting, as the treatment of overshadowing of allotments and growing space shows: what the space is for changes which hours of the day matter.

Single storey against two storey

The choice between one and two storeys on a back land plot is usually made by the shadow study rather than by the development appraisal, and accepting that early saves a refusal.

Single storey with a flat or shallow roof, set below the boundary wall line where levels allow, is the form that passes most often. A mono-pitch rising away from the most sensitive boundary does nearly as well and gives better internal volume.

Two storeys can work in three situations. Where the plot sits north or east of the affected gardens. Where the gardens are deep enough that a 10 metre shadow still leaves most of the area in sun. And where the existing condition is already heavily obstructed, so the ratio test is satisfied even though the absolute figure is poor.

Lowering the floor level to reduce the ridge is tempting and needs care. It changes the drainage strategy, may bring the dwelling into a surface water flow path, and can require a pumped foul connection. Check those before you draw it.

The new dwelling's own garden

Officers refuse back land schemes on the amenity of the proposed dwelling at least as often as on the neighbours'.

Your own garden is surrounded by the same walls and the same neighbouring buildings. Run the sun on ground test on the proposed garden too, and if it fails, say what you have done about it: a south-facing garden rather than a side strip, a usable terrace at the sunniest corner, generous glazing to the living room on the right elevation.

Internal space matters alongside it. Check the proposed floor areas against the nationally described space standard, because a cramped plan plus a shaded garden is the combination that produces a refusal on living conditions, and neither alone would.

Local plan amenity policies and how they use BRE 209 vary widely on whether garden sunlight is quantified at all, so read the adopted wording and any householder design guide before you decide how much analysis to commission.

What to put in the application

Six items, and the pack is short.

• A measured survey of the site and the boundaries, with wall and fence heights and neighbouring building heights.
• Sun on ground plots for the existing and proposed conditions at 21 March, with 21 June and 21 December for context.
• A section through the most sensitive boundary showing the shadow angle at the equinox.
• Window positions on the proposed dwelling with sill heights, to answer overlooking before it is raised.
• A short planning statement dealing with garden land policy, living conditions for the neighbours and for the new occupier, and access.
• The access and refuse arrangements, because on back land sites they generate as many objections as light does.

Daylight and sunlight work is an area where councils expect a suitably qualified author, and professional review is strongly advised before submission. Enter your postcode on PlanScope to see which reports a back land plot is likely to need.

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.