Assessing daylight inside the homes you are building

Internal daylight to the flats you are building is now assessed as closely as the loss of daylight to the neighbours, and on a dense urban site it is the test more schemes fail. The third edition of BRE 209 brought the internal recommendations into line with BS EN 17037, so the question has shifted from a single average daylight factor per room to whether a stated illuminance is achieved over a defined part of the room for a defined part of the year.

That change matters commercially. A plan that passed on average daylight factor can fail on target illuminance, because the new method cares where in the room the light falls, not only how much arrives.

Why internal daylight is now assessed

Three things drove it. National policy expects a good standard of amenity for existing and future occupiers. Local plan policies on housing quality, and in London the London Plan's housing quality and standards policy, give officers a hook to ask for the evidence. And the pattern of recent schemes, with deeper plans, more single aspect units and more inset balconies, made the issue visible.

Officers also learned that neighbour impact and internal provision can point in opposite directions. Pulling a block back from a boundary to protect an existing window makes the new units deeper for the same floor area, which worsens their own daylight. The two assessments belong in one document for that reason, and the approach local plans take to the guidance is worth checking first, which is the subject of local plan amenity policies and how they use BRE 209.

The two methods and which one to use

BS EN 17037 permits two routes to assessing daylight provision.

The first is climate-based daylight modelling, using an hourly climate file for the location to calculate illuminance on a reference plane across the year, then reporting the fraction of the plane meeting a target illuminance for at least half of the daylight hours. This is the method the standard presents first and the one most consultants now run.

The second is the daylight factor method, which converts the target illuminance into a daylight factor using the diffuse illuminance available at the location, and is simpler to compute and to check. Results from the two are related but not identical, and a scheme close to the line can pass on one and fail on the other.

Many authorities still ask for average daylight factor against the older BRE recommendations of 1 per cent for bedrooms, 1.5 per cent for living rooms and 2 per cent for kitchens. Where a validation list or a local policy says that, provide it. Then provide the current assessment alongside, with a short note explaining the relationship, rather than arguing that the request is out of date.

Whichever route you take, state the inputs: the climate file, the surface reflectances, the glazing transmittance, the maintenance factor, the obstruction model and whether balconies and reveals are modelled in geometry or approximated.

Targets by room type

The UK National Annex to BS EN 17037 sets minimum target illuminances that differ by room use, because the tasks carried out in each room differ:

The target has to be achieved over at least half of the reference plane for at least half of the daylight hours. The reference plane is normally taken as a horizontal plane 0.85 metres above the floor with a border excluded adjacent to the walls, so a long thin room with a window at one end can meet the target across the window half of the plane and fail across the rest.

Combined rooms are the usual argument. An open plan kitchen, living and dining space is assessed against the kitchen figure in some authorities and against a weighted or zoned approach in others, and a plan that puts the kitchen at the back of a deep space is the hardest case. Agree the convention with the officer before running the model, and if you adopt a zoned approach, show the zones on the plan.

Where single aspect plans fail

The failures cluster. North-facing single aspect units in a deep plan, ground floor units with an obstruction opposite, and units whose only window opens into a recess all produce the same result for different reasons.

Depth is the dominant variable. Beyond roughly two and a half times the head height of the window, the far end of a room receives very little sky light, and no glazing specification recovers it. Widening the window helps the near zone more than the far zone. Raising the head height helps the far zone, which is why a higher floor to ceiling dimension does more for daylight than a wider window on a deep plan.

The remedies that work are structural rather than cosmetic: reduce the depth, win a second aspect at a corner, split a deep unit into two shallower ones, or raise head heights and lower cill heights together. The options and their cost consequences are set out in daylight to single aspect flats and how to fix them.

And be honest in the submission about the units that fail. A report that claims full compliance across 140 flats invites scrutiny of the model. One that identifies nine units falling short, explains why, and sets out what was done to limit the number, reads as competent.

Balconies, reveals and light wells

An inset balcony above a window removes a large part of the visible sky from that window. The effect is geometric and severe, and modelling it properly rather than as an approximation is the difference between a defensible report and an optimistic one. Projecting balconies with open balustrades cost less daylight than inset ones, and a glazed balustrade still has a frame.

Deep reveals do the same at smaller scale. A 300 millimetre reveal on a window 1.2 metres wide takes a measurable bite out of the sky view, and reveals that deep are common where the facade carries an insulated render build-up or a brick slip system.

Light wells and courtyards need checking at the lowest floor, not the average. A courtyard that works at level four can be hopeless at level one, and the fix is usually to splay the reveals, widen the court at the base, or change the use of the lowest units. Reflective finishes in a court help a little and are often oversold, so model the reflectance you will actually specify and maintain rather than a laboratory value.

The trade off with overheating

Every change that improves daylight increases glazing area or sky view, and both increase solar gain. Part O of the Building Regulations limits overheating risk, and on a south or west facing single aspect flat with limited cross ventilation the two requirements pull hard against each other.

The resolution is usually external shading that is selective rather than blanket: brise soleil on the upper part of the window, a deeper reveal on the west elevation only, or a balcony positioned to shade in summer. Blanket solar control glazing reduces daylight across the whole year to solve a summer problem, and the daylight model then fails.

Run the two assessments together on a sample of worst-case units early, before the facade proportions are fixed. Running them sequentially produces two reports that contradict each other and a redesign at technical design stage.

Reporting room by room

A schedule, not a narrative. The structure officers can actually review:

1. A unit schedule listing every habitable room with floor, aspect, room type, area, window area and head height.
2. The target illuminance applied to each room and the basis for it.
3. The fraction of the reference plane meeting the target, and the result against the criterion.
4. A plan per floor with rooms shaded by result, so the pattern of failure is visible at a glance.
5. A short section on the units that do not meet the target, with the reason and the mitigation considered.
6. The full input assumptions in an appendix.

Keep the neighbour assessment in the same Daylight and Sunlight Assessment document so the two sets of results can be read against each other, and deal with amenity space separately through an Overshadowing Study where the scheme has gardens or a communal courtyard.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Daylight modelling is work councils expect from a suitably qualified author, and professional review is strongly advised.

To see which amenity and environmental reports a scheme is likely to need, enter your postcode on PlanScope and work through the constraints it detects. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.