Assessing daylight for a proposed building's own rooms
Neighbour tests measure what your building takes away. Internal daylight asks whether the rooms you are proposing work at all, and it is judged against different numbers.
Internal daylight in new dwellings is now assessed against BS EN 17037 and its UK National Annex, which set target illuminances rather than a single daylight factor. For dwellings the National Annex gives minimum targets of 100 lux in bedrooms, 150 lux in living rooms and 200 lux in kitchens, to be met over at least half the assessment area for at least half the daylight hours in the year. That is a different exercise from the neighbour tests, uses different software settings, and produces a different kind of answer.
Most planning objections are about the neighbours. Most refusals on housing quality are about the rooms you are proposing. The two need to be in the same report and kept apart within it.
Internal daylight against neighbour impact
The neighbour assessment asks what your building does to existing windows. Vertical sky component, the no-sky line and annual probable sunlight hours all measure the sky or sun available to a window that already exists, and the benchmarks in BRE 209 are about the change from the existing condition.
The internal assessment asks whether the rooms you are building will be adequately lit. There is no baseline to compare against. The room either meets the target or it does not.
Practically, this means you cannot answer an internal daylight objection with a neighbour result, and consultants do sometimes try. A scheme can pass every vertical sky component test on the street and still produce flats that fail on illuminance, because the obstruction limiting your own rooms is usually your own building across the courtyard.
Run both, report both, and put the internal results first when the application is for new homes. The relationship between the two standards is set out more fully in our note on BS EN 17037 targets against BRE guidance.
Daylight factor and illuminance based methods
BS EN 17037 allows two routes to the same target.
The daylight factor method converts the target illuminance into a required median daylight factor using the median external diffuse illuminance for the location. Because that external figure varies with latitude and climate data, the resulting daylight factor target is not a single national number, and reporting a bare daylight factor without stating the location and the conversion is not enough.
The climate-based route models the space against a full year of hourly weather data for the nearest representative location, and reports the proportion of the assessment area achieving the target illuminance for the required fraction of daylight hours. It is more work, it is more realistic, and it handles orientation properly, which the daylight factor method does not.
Some authorities still ask for average daylight factor against the older BRE figures of one, one and a half and two per cent for bedrooms, living rooms and kitchens. Where the validation list says so, provide it, and provide the BS EN 17037 results alongside. What you must not do is quote an average daylight factor and describe it as compliance with BS EN 17037. They are different measures and a competent reviewer will say so. The changes the 2022 edition introduced are covered in our note on what BRE 209 changed for new interiors.
Spatial daylight measures
The move from an average to a spatial measure is the part that catches designers out.
An average rewards a bright strip by the window even where the back of the room is dark. A spatial measure asks how much of the room reaches the target, so a deep room with a single window fails however bright the front half is.
The assessment area is defined on a horizontal reference plane, conventionally 0.85 m above floor level in dwellings, with a border adjacent to the walls excluded. Two consequences follow. Room depth is penalised directly, and the shape of the room matters as much as the glazing area.
| Design variable | Effect on a spatial measure |
|---|---|
| Deeper room, same window | Worse, and the penalty grows with depth |
| Taller window head, same area | Better, light reaches further back |
| Wider window, same area | Modest gain, mostly near the window |
| Higher ceiling with raised head | Better |
| Lighter internal surface finishes | Real but small, and you must model the actual values |
| Deep external reveal | Worse, and often forgotten in the model |
Raising the window head is the most efficient single move. It costs nothing in glazing area and it changes the geometry of the whole room.
Single aspect and north facing units
A single-aspect unit has one window wall and therefore one daylight opportunity. Facing north, it also receives no direct sun.
The London Plan expects proposals to maximise dual-aspect dwellings and to avoid single-aspect dwellings that are north facing. Plenty of authorities outside London carry similar policies, sometimes with a numerical cap on the proportion of single-aspect units in a scheme.
Where single aspect is unavoidable on a deep plan, the moves that help are architectural rather than technical. A wider frontage per unit with a shallower plan. A recessed corner giving a second orientation. Relocating bathrooms and stores to the internal core so every habitable room is on the perimeter. A shorter block with a through-flat at each end.
Report the number and proportion of single-aspect and north-facing single-aspect units explicitly. Officers will count them from the plans if you do not, and being the one who supplies the figure is better than being the one who appears to have hidden it.
Balconies and their effect on the rooms behind
A balcony is a horizontal obstruction directly above the window it shades, which is the worst possible position. On a deep inset balcony the room behind can lose a large share of its daylight, and inset balconies on the lower floors of a courtyard block are where schemes fail.
Options, roughly in order of effectiveness: project the balcony rather than inset it, reduce its depth, use an open mesh or glazed soffit and a glass balustrade, offset it sideways so it sits beside the main window rather than over it, or accept it and raise the window head to compensate.
Model the balcony as built, with the balustrade material and the soffit. A model that omits the balustrade produces results the built scheme will never achieve, and if the omission is spotted the whole assessment loses credibility.
The same discipline applies to brise soleil, projecting fins and deep reveals. Anything that shades the window belongs in the model.
Where a local plan sets internal standards
National policy does not set a numerical internal daylight standard. Local policy sometimes does, and where it does it governs.
Check three documents before fixing the method: the adopted local plan housing quality policy, any residential design supplementary planning document, and the validation checklist. Some authorities specify average daylight factor with target values. Some specify BS EN 17037. A few set their own room-depth or window-to-floor-area ratios instead, which are cruder and easier to design to.
Where the authority is silent, use BS EN 17037 with the UK National Annex and say why. An assessment that names its standard, its method, its software and its assumptions is defensible even where the authority would have preferred a different one.
Non-residential buildings sit under different expectations again, which our note on daylight and sunlight for schools and healthcare buildings covers.
Reporting internal and external results together
One report, clearly divided, with a schedule the officer can check room by room.
- State the method, the standard, the software, the sky model, the weather file and the surface reflectances used, in one place near the front.
- Present the internal results as a table of rooms against targets, with the assessment area shown on a floor plan and failures identified rather than aggregated away.
- Present the sunlight exposure results separately, noting which habitable rooms have a main window facing within 90 degrees of due south.
- Present the neighbour results in their own section, against the BRE 209 benchmarks, with the existing and proposed values side by side.
- Close with a short statement on the units that fall short, why, and what design response was tested.
That last item is what distinguishes a report an officer can rely on. A scheme of any density will have some rooms below target. Naming them, quantifying the shortfall and explaining the design constraint is a stronger position than a document claiming universal compliance that a reviewer then picks apart. Councils normally expect a daylight and sunlight assessment from a suitably qualified specialist, and professional review is strongly advised.
Enter your postcode on PlanScope to see which reports a residential scheme on your site is likely to need.
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.
Frequently asked questions
What target illuminances apply to new dwellings?
The UK National Annex to BS EN 17037 gives minimum targets of 100 lux in bedrooms, 150 lux in living rooms and 200 lux in kitchens. Each must be achieved over at least half the assessment area for at least half the daylight hours in the year, measured on a horizontal reference plane within the room.
Can I still use average daylight factor?
Where the validation list or local plan asks for it, yes, using the older BRE figures of one, one and a half and two per cent for bedrooms, living rooms and kitchens. Provide the BS EN 17037 results alongside. Do not present an average daylight factor as compliance with BS EN 17037, because they are different measures.
Why does room depth matter so much now?
Because the standard uses a spatial measure rather than an average. A deep room with one window can be bright near the glass and dark at the back, which an average rewards and a spatial measure penalises. Raising the window head is usually the most efficient response, since it costs no additional glazing area.
Are single aspect north facing flats acceptable?
The London Plan expects proposals to maximise dual aspect dwellings and to avoid single aspect dwellings that are north facing, and many authorities elsewhere carry similar policies. Where single aspect is unavoidable, report the number and proportion openly, because officers will count them from the plans in any case.
How should balconies be modelled?
As built, including the balustrade material, the soffit and any brise soleil or projecting fins. A balcony is a horizontal obstruction directly above the window it shades, and inset balconies on lower floors of a courtyard block are a common failure point. Omitting the balustrade produces results the building will never achieve.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.