Daylight targets for new rooms and how they are tested

The daylight in new dwellings target most consultants now work to is an illuminance figure, not a daylight factor. BS EN 17037, with its UK National Annex, sets recommended minimum target illuminances of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, to be achieved over at least half the area of the reference plane for at least half the daylight hours in the year. That is a different exercise from the average daylight factor most of us grew up with, and the switch changes which rooms pass.

The tests for existing neighbouring windows have not changed. Those remain the vertical sky component, the no-sky line and annual probable sunlight hours in the BRE guidance. This article is about the proposed dwellings.

Targets for new rooms against tests for existing ones

Keep the two apart in your head and in the report.

Assessment of effect on neighbours is a comparison: before and after, with numerical criteria for when a loss is noticeable. Assessment of daylight inside your own new units is an absolute standard: does the room meet a target, yes or no.

They also use different inputs. The neighbour tests care about the obstruction your building creates. The internal targets care about window size, room depth, glazing transmittance, internal surface reflectance, balcony overhangs and the obstruction outside, including your own other blocks.

The consequence for design is that a scheme can perform well on neighbour impact and badly on internal daylight, and the second failure is usually harder to fix, because it is a function of the building's own depth and section.

The illuminance method and the hours it uses

BS EN 17037 offers two routes to the same recommendation.

The illuminance method uses climate-based daylight modelling with hourly weather data for the location, and reports the fraction of daylight hours in the year for which the target illuminance is met across the reference area. The daylight hours in the standard are the hours between sunrise and sunset, so no artificial lighting assumption is involved.

The daylight factor method converts the target illuminance into a target daylight factor using the median external diffuse illuminance for the location, then assesses against that. It is faster and it is the route most reports take, and it is a legitimate application of the standard rather than a shortcut.

Two recommendations run together. The target illuminance has to be met over at least 50 per cent of the reference plane area, and a lower minimum target illuminance has to be met over at least 95 per cent of it. The second limb is what catches deep rooms with a bright zone at the window and a dark back half.

The reference plane sits at a stated height above the floor, set back from the walls. How that plane is defined in a model materially changes the result, so the report should state it.

Where average daylight factor survives

Plenty of local plans, supplementary planning documents and borough housing design guides still ask for average daylight factor values of 1 per cent for bedrooms, 1.5 per cent for living rooms and 2 per cent for kitchens. Those figures come from the withdrawn British Standard that BS EN 17037 replaced.

A withdrawn standard cited in an adopted policy is still what the policy asks for. So the pragmatic answer on many schemes is to report both, and to be explicit about which document each figure comes from.

Where the two disagree, say which rooms pass under one and fail under the other, and explain why. The usual pattern is that the new standard is more demanding for kitchens and living rooms in deep plans, and the treatment of rooms combining a kitchen, dining and living function is where the arguments concentrate. Our note on room use assumptions and how they change results covers that in detail.

Room by room targets by use

The open-plan combined space is the practical difficulty. A room serving as kitchen, dining and living space has to be assessed for something, and the choices are to apply the kitchen target to the whole space, to apply it only to the kitchen zone, or to apply the living room target on the basis that the kitchen is a subsidiary function.

Authorities differ, and a few state their preference in a design guide. Where none does, apply the kitchen target to the kitchen zone with the zone marked on the plan, report the whole-room result as well, and say which you consider the correct reading. A report that quietly applies the lowest applicable target to the largest area is the one an objector's consultant takes apart.

Balconies, reveals and overhangs in the model

These are where reported results and built reality diverge.

A balcony above a window is the single most effective way to lose daylight in a flatted scheme. An inset balcony with a solid soffit over a full-width glazed wall can halve the light reaching the back of the room, and the effect is worse on lower floors where the sky view is already restricted.

Deep window reveals matter, and so does the frame. A model that treats the window as a glazed hole in a zero-thickness wall overstates performance. So does one that uses nominal glazing transmittance rather than the specified glass, which on a triple-glazed unit with a solar control coating can be materially lower than assumed.

Internal surface reflectances are the other lever, and the one most open to challenge. A model assuming high reflectance for walls, ceiling and floor produces better numbers than a room that will be decorated in a dark colour with a wood floor. State the values used, and use values you would be comfortable defending.

Three modelling decisions to record explicitly: the reflectance set, the glazing transmittance, and whether balustrades were modelled as solid, perforated or absent.

Single aspect and north facing units

A single aspect north-facing flat can meet the illuminance targets and still be a poor unit, because the standard measures quantity of light rather than sunlight, view or the ability to cross-ventilate.

Policy addresses this separately. The London Plan's housing quality policy expects schemes to avoid single aspect dwellings that are north facing, and many local plans outside London carry similar wording. A daylight report that says every unit complies, in a scheme with a run of north-facing single aspect flats, is answering a question the officer did not ask.

BS EN 17037 also contains recommendations on sunlight exposure, view out and protection from glare. The sunlight recommendation is expressed in hours on a date in the late winter or early spring period, and reporting it alongside the daylight results gives a much fuller picture of what living in the unit is like.

There is a real tension with overheating here. Larger windows improve daylight and worsen summer overheating under Approved Document O and CIBSE TM59, and the mitigation for overheating, which is shading, reduces daylight. Running the daylight model and the Overheating Risk Assessment in parallel rather than in sequence saves a redesign.

Reporting compliance and non-compliance honestly

BS EN 17037 uses the language of recommendations rather than requirements, and it sets levels of minimum, medium and high. Say which level you assessed against.

A report that helps the decision-maker contains a unit-by-unit schedule with room uses named, the target applied to each, the result, and a clear marker for every room that falls short. Summarise as a percentage of rooms and units meeting the target, but do not summarise instead of scheduling.

Where rooms fall short, explain why in design terms: plan depth, an inset balcony, an obstruction from the adjacent block, a basement level courtyard. Then say what was tested to improve it and what the trade-off was. A scheme where 12 per cent of habitable rooms fall short, all of them bedrooms on the lowest two floors of a courtyard block, is a comprehensible planning judgement. The same shortfall presented as a single compliance percentage with no explanation is not.

Councils expect a suitably qualified author for daylight work, and professional review of any draft is strongly advised before submission. For what to test at massing stage, before the units exist, see obstruction angles and the early massing test, and for the presentation stage, presenting daylight results a committee can read. The scope of a full Daylight and Sunlight Assessment sets out what a commission normally covers.

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.