Sunlight to balconies and private terraces

Sunlight to balconies in planning terms has no criterion of its own. The Building Research Establishment guidance on site layout planning for daylight and sunlight sets a sun on ground recommendation for gardens and amenity areas, which is that at least half the area should receive at least two hours of sunlight on 21 March, and it was written for ground level space. Applying that to a balcony four storeys up is an adaptation, not a compliance test, and saying so in the report is what separates a defensible assessment from one an objector can take apart.

Whether the sun on ground test applies to a space above ground level

The guidance's own framing is gardens and open spaces. Private amenity space above ground level was not what the criterion was derived for, and nothing in it tells you how to treat a 1.5 m deep slot between two party walls.

In practice most assessors run the same two hour test on 21 March across the balcony surface and report the proportion of the balcony area that meets it. That is reasonable, and it gives a comparable number across a scheme. What it is not is a pass or fail against a published standard, and a report that presents it as one invites the obvious question at committee.

Two things make the adaptation harder than it looks on a garden. A balcony is small, so the result is sensitive to a few hundred millimetres of soffit depth. And a balcony is enclosed on up to three sides, so the sky view is narrow and the sun reaches it for a short window even when the orientation is favourable.

State the method, state its origin, state that it is applied by analogy. Then give the numbers.

Inset balconies and the room behind them

The more serious issue is not the balcony. It is the living room behind it.

An inset or recessed balcony puts a solid overhang directly above the window and solid returns either side of it. The room loses sky view across the whole of its glazed area, and on a single aspect flat that is the only daylight the room will ever have. The daylight result for such rooms is routinely poor, and the balcony is the cause.

The analysis that answers this is to model the room twice, once with the balcony as designed and once with the recess removed, and report both. That shows the authority exactly what the balcony is costing in daylight terms and allows a reasoned judgement about the trade between internal light and outdoor space. It is also the analysis that supports a design change, because it quantifies the gain from moving the balcony to a projecting position or reducing its depth.

Single aspect flats with inset balconies are the worst combination in residential layout, and the remedies, from dual aspect corner planning to reducing the room depth, are set out in our note on daylight to single aspect flats.

Assessing a terrace as usable amenity space

A roof terrace behaves more like a garden than a balcony does, which makes the sun on ground test a better fit, with one caveat. Terraces are often bounded by parapets, plant enclosures, lift overruns and privacy screens, and those obstructions sit close to the surface being tested.

Model them. A 1.1 m parapet casts a long shadow across a shallow terrace in March, and omitting it produces a result that will not survive a site visit in winter.

Where a terrace is communal rather than private, the amenity argument changes. Communal space can take a lower proportion of sunlit area and still function, because residents move around it, and the London Plan requirement for private outdoor space is separate from the provision of communal amenity space. In Greater London, Policy D6 of the London Plan sets a minimum of 5 m² of private outdoor space for a one or two person dwelling, with 1 m² for each additional occupant and a minimum depth and width of 1.5 m, so a balcony that fails the sunlight test may still be required for its area.

Orientation and the hours that are actually useful

A north facing balcony in England receives no direct sun at all for most of the year. No amount of balustrade specification changes that, and presenting it as a mitigation question wastes everybody's time.

East facing space gets morning sun. West facing space gets late afternoon and evening sun, which residents use more, and which the two hour test on 21 March treats identically. That is a limitation of the test worth naming in the report rather than hiding behind the percentage.

South facing inset balconies have the opposite problem. They perform well on the sunlight test and badly on overheating, because the room behind them has a southern aspect with limited cross ventilation. The design that wins the sunlight argument can lose the overheating one, and both are assessed at planning stage on larger schemes.

Balustrades, soffits and light transmission

Three construction decisions change the result more than the orientation does.

Specify the balustrade transmittance and the soffit reflectance in the assessment rather than leaving the modeller to assume a default. Those two values move the internal daylight results enough to change a conclusion, and a condition requiring materials in accordance with the approved details is how the authority holds you to them.

When a Juliet balcony is the better answer

On a deep plan single aspect flat, a Juliet balcony behind a full height opening usually produces a better home than an inset balcony does. The room keeps its sky view, the daylight result improves, and the external space is lost.

That is a real loss, and in London it may put the scheme below the Policy D6 private outdoor space requirement, which then needs addressing elsewhere in the layout. Outside London, local plan standards vary and some authorities have no numerical requirement at all, so check the adopted policy before assuming the trade is available. How local plans use the Building Research Establishment guidance differs more than most applicants expect, and our note on local plan amenity policies and BRE 209 sets out the common variations.

The honest position is that some urban sites cannot deliver both good internal daylight and compliant private outdoor space in every unit. Say which you have prioritised and why.

Reporting the analysis so it survives scrutiny

A balcony and terrace sunlight assessment should contain the method and its source, the date and sun position data used, a drawing identifying every balcony and terrace assessed with a reference, the modelled obstructions including parapets and screens, the balustrade and soffit assumptions, results per space with the proportion meeting the two hour criterion, and a statement of limitations.

Report the failures. A scheme where eight balconies out of sixty fall below the criterion, with reasons and with the design choices that caused them, reads as competent. A scheme where everything passes reads as a model that was adjusted until it did.

The analysis usually sits inside a wider Daylight and Sunlight Assessment covering internal daylight, sunlight to windows and effects on neighbours, and where neighbouring gardens are affected a separate Overshadowing Study deals with the ground level transient shadow. Councils expect this work from a suitably qualified author, and professional review is strongly advised before anything is submitted.

If you are scoping the evidence base for a residential scheme and want to see which documents the site is likely to need, enter the postcode on PlanScope and work through the constraints first.

This is general guidance, not legal advice, so 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.