Average daylight factor targets for new flats

For new flats the working daylight targets are still an average daylight factor of 2 per cent in a kitchen, 1.5 per cent in a living room and 1 per cent in a bedroom. Those figures came from BS 8206-2, they are carried through in BRE 209, and most English planning officers assess residential daylight against them even though the British Standard that set them has been withdrawn in favour of BS EN 17037.

That overlap is the source of most arguments on submitted assessments. Two methods, two sets of numbers, and a policy that often names neither. Getting the reporting basis agreed before modelling starts saves a round of revisions.

What average daylight factor measures

Average daylight factor is the ratio of internal illuminance to unobstructed external illuminance under a standard overcast sky, averaged across the working plane of a room and expressed as a percentage. It is a geometric calculation, not a climate one.

The inputs are few: net glazed area, visible light transmittance of the glazing, total internal surface area of the room, average surface reflectance, and the angle of visible sky from the centre of the window. Because it uses a standard overcast sky, it says nothing about orientation. A north-facing and a south-facing room with identical geometry return the same average daylight factor, which is both the method's convenience and its main weakness. The consequences for a north-facing room are set out in daylight to a north-facing habitable room.

Room-type targets and what they assume

The targets are not arbitrary. They reflect an assumption about how much of each room's use depends on daylight and how long people occupy it.

Two practical points follow. In an open-plan kitchen, living and dining space the higher kitchen target applies to the whole room, which is why open-plan layouts fail more often than cellular ones in the same building. And a room labelled as a study or a snug on the drawings is assessed as a habitable room, so labelling will not rescue a poorly lit space.

Where a shortfall is unavoidable, the honest approach is to report it by room with the figure, rather than averaging across the flat until it disappears.

How BS EN 17037 criteria differ

BS EN 17037 works in illuminance rather than daylight factor. It asks whether a target illuminance is achieved across a proportion of the reference plane for a stated proportion of daylight hours, and the UK National Annex sets room-specific target values for dwellings that are lower than the general European targets.

Three differences matter in practice:

1. It is climate-based, so orientation, local weather data and obstructions all affect the result. Two identical rooms facing different directions give different answers.
2. It assesses a proportion of the area rather than an average, so a room can pass on the average daylight factor method and fail the area-based criterion, or the reverse.
3. It brings in view out and sunlight exposure as separate criteria, which the average daylight factor method does not address at all.

Because it is climate-based, the method rewards good orientation and penalises a deep plan with a single north window more honestly than average daylight factor does. It is also more sensitive to modelling assumptions, which is why a reviewer will want the weather file and the schedule of daylight hours stated.

Ask the local planning authority which basis it wants at pre-application. Some boroughs now ask for both, which is a reasonable request and roughly doubles the modelling fee.

Glazing area, reflectance and room depth

The lever with the most effect is usually room depth, not glazing area. Beyond roughly two and a half times the window head height, additional depth adds floor area that receives very little daylight while increasing the total internal surface area in the denominator. So a deep room can get worse when you make it bigger.

Glazing specification matters more than people expect. Visible light transmittance for a double-glazed unit with a solar control coating can sit well below that of a clear unit, and a triple-glazed unit lower still. Where the overheating strategy is pushing towards a low g-value glass, the daylight calculation and the Part O assessment are pulling against each other, which is the trade-off covered in daylight, glazing area and Part O overheating.

Surface reflectance is a legitimate input and a common place to overclaim. A model assuming very high wall and ceiling reflectance across every room is assuming a decorating scheme nobody has committed to. Use reasonable values, state them, and do not rely on reflectance to close a large gap.

Balconies and overhangs in the calculation

Inset balconies are the single most common cause of daylight failure in new flats. A balcony recessed into the building over the window below removes most of the visible sky from the room behind it, and the loss is severe enough to take a compliant room to less than half the target.

The effect is geometric and predictable, so it can be designed out:

1. Project the balcony beyond the facade rather than insetting it, where massing and overlooking allow.
2. Keep the balcony to one side of the window rather than across its full width.
3. Use an open balustrade and an open or partly open soffit rather than a solid slab.
4. Where an inset balcony is unavoidable, put it outside the most daylight-sensitive room, which usually means a bedroom rather than a kitchen.

Projecting canopies, brise soleil and deep window reveals all work the same way at a smaller scale. Model them as built rather than as a flat facade, because an assessment that ignores a 1.2m overhang will be rejected on review.

Reporting by unit and by room

Report every habitable room in every unit, not a sample. Where a scheme has hundreds of flats, the schedule can be an appendix, but the summary needs to state how many rooms meet the target, how many fall short, and by how much.

A useful summary distinguishes three groups: rooms meeting the target, rooms marginally below it, and rooms substantially below. Officers and committee members can work with that. A single sentence saying most rooms comply invites a request for the full schedule and another three weeks on the determination clock.

Include the assumptions on the face of the report: glazing transmittance, reflectances, whether balconies and reveals are modelled, the working plane height, and which rooms are treated as kitchens. The Daylight and Sunlight Assessment has to be reproducible by a peer reviewer, and most of the disputes that reach appeal are about assumptions rather than arithmetic.

Arguing an urban context shortfall

BRE 209 describes itself as guidance and says its numerical criteria are advisory rather than mandatory, which leaves room for a context argument. Room for it, not a free pass.

An argument that works has three parts. Evidence of what daylight levels existing flats in the same street pattern actually achieve. A design that demonstrably does everything reasonable within the constraint, meaning balconies resolved, room depths controlled and glazing maximised. And a clear statement of the residual shortfall by room, with the worst cases named.

An argument that fails leads with density policy and treats the numbers as a formality. The detail of what a dense site can realistically reach is covered in daylight in a dense urban block.

Enter your postcode on PlanScope to see which daylight and amenity reports your scheme is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified daylight consultant 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.