Balconies and recessed windows in daylight assessments

A balcony daylight assessment usually goes wrong in the same place: the daylight modelling is run on the base building geometry and the balconies are added to the elevations afterwards. An overhead balcony slab removes a large part of the visible sky from the window beneath it, and on a recessed unit with a single aspect that alone can move a room from passing to failing. On a residential scheme where private amenity space policy pushes towards balconies on every unit, the two requirements pull against each other and somebody has to resolve it before the planning drawings are frozen.

Get the geometry into the model early. It is cheaper to move a balcony at concept stage than to argue about a daylight failure at committee.

Why an overhead balcony cuts sky access

Daylight to a room depends on how much sky the window can see. A slab projecting 1.8m above a window occupies the upper part of the view, which is the part carrying most of the useful light, because the sky is brighter overhead than near the horizon. Losing the top of the visible sky costs more than losing the same angular area near the ground.

The effect compounds with floor height. On a low floor in a courtyard, the window is already looking at the opposite building for much of its view, and the balcony above removes what little sky remained. The top two floors of the same stack may be unaffected. That is why balcony daylight problems appear as a band of failing units rather than as a uniform reduction, and why a scheme-wide average conceals them.

Where the surrounding massing is also constraining the view, the balcony and the context interact, and the early testing described in obstruction angles and the early massing test is the right stage to catch it.

Inset and projecting balconies compared

An inset balcony sits within the building envelope, so the room behind it looks out through a space with walls on two or three sides and a slab above. A projecting balcony cantilevers beyond the facade, so the room's window is at the facade line and only the slab above obstructs it.

In daylight terms projecting is almost always better, and the gap is often large. The trade-offs run the other way: projecting balconies are harder to detail for thermal bridging, they affect the facade's appearance and the neighbouring overlooking picture, and in some contexts an officer will resist them on townscape grounds.

Corner and part-width balconies

Part-width balconies are the most useful tool in the set, and the most often overlooked. If the balcony occupies half the width of the glazing and the other half is left open to the sky, the room keeps an unobstructed path to the upper sky over part of its aperture, and the assessment result improves markedly for a small loss of amenity area.

Corner balconies behave similarly. A balcony wrapping a corner gives one facade a clear view while obstructing the other, and where the plan allows the living space to sit against the clear facade the result can be better than a symmetrical arrangement.

Neither is a trick. Both are ordinary design responses that require the daylight consultant and the architect to be in the same conversation while the plan is still soft.

Winter gardens and glazed enclosures

A glazed winter garden is offered as a compromise where noise or wind rules out an open balcony. The daylight consequence depends on the glazing specification, the frame proportion and whether the enclosure is openable.

Two cautions. First, the light transmission of the glazing and the frame area both reduce daylight to the room behind, and a heavily framed enclosure can be worse than an open balcony with a slab above. Second, an enclosed space may count towards internal floor area under national space standards or towards the amenity space requirement, or towards neither, depending on the authority and the policy wording. Ask the case officer, and where the scheme is in London check the borough's reading of the relevant London Plan policy rather than assuming.

Overheating is the third consideration. A glazed enclosure on a south facade changes the Approved Document O and CIBSE TM59 picture for the room behind it, which ties this decision to the Overheating Risk Assessment rather than leaving it in the daylight report alone.

Testing options before the elevation is fixed

The efficient sequence is a coarse test on several options rather than a precise test on one.

1. Model the base massing with no balconies, and identify which units are close to the relevant target.
2. Add the proposed balcony geometry and re-run, to see the size and distribution of the loss.
3. For the affected band, test part-width, projecting and reduced-depth variants.
4. Fix the balcony strategy per floor band rather than applying one rule to the whole elevation.
5. Only then run the detailed assessment that goes into the Daylight and Sunlight Assessment.

Step four is where schemes gain the most. There is no requirement for every floor to have the same balcony arrangement, and a scheme with projecting balconies on the lower three floors and inset above reads perfectly well on an elevation.

Illustrative example. On a seven-storey block facing a 16m street, the base model shows the first and second floor living rooms sitting just above the target. Adding full-width inset balconies 2m deep pushes both floors below it. Switching those two floors to projecting balconies with a 1.5m depth, and keeping inset balconies from the third floor up, restores the lower units while keeping the amenity area within policy. The figures here are illustrative and specific to the geometry described.

Trade-offs with amenity space policy

Most authorities set a minimum private amenity area per dwelling, often with a size varying by bedroom count, and in London the London Plan sets a baseline that boroughs apply. Reducing a balcony to solve daylight can breach that minimum, and the answer is not to shave both until each is marginal.

The workable responses are to redistribute rather than reduce: deeper balconies on upper floors where daylight is not constrained, ground floor terraces or gardens for lower units, and shared amenity space making up the difference where the policy permits substitution. Where shared space is relied on, its own sunlight performance matters, which is the subject of courtyard blocks and sunlight to shared amenity space.

Explaining the design choice in the statement

Officers accept a compromise they can see was made deliberately. What they reject is a daylight report that reports failures without explanation and a design statement that claims balconies for every unit as a benefit.

Say which units fall below the target, by how much, and why the arrangement chosen is the best available. State the options tested and the reason each was set aside. Where a unit remains below target, explain the compensating factors honestly: dual aspect, a generous window, a quiet outlook, a larger room. And be direct about the one thing readers of these reports look for, which is whether the applicant understood the trade-off or discovered it late.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Daylight and sunlight assessments are normally expected to be prepared by a suitably qualified specialist, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.