Worked example: internal illuminance target check for a flat
Climate based daylight modelling reports whether a target illuminance is met across half the room for half the daylight hours. Here is how the calculation is set up and what an illustrative single aspect flat produces.
A climate based daylight check asks one question: does a given illuminance fall on the reference plane across at least half the room area, for at least half the daylight hours in the year. Pass or fail against that target is what a case officer or a design review panel is looking for, and the UK national annex to BS EN 17037 sets the target illuminance values that apply to homes. A single aspect flat behind a balcony is the arrangement that most often fails, and the worked numbers below show why.
The method replaced the average daylight factor approach that architects used for two decades. It is not a refinement of it. A daylight factor is a ratio under an overcast sky, while a climate based calculation runs an hourly weather file for the location and counts the hours the target is actually met.
What a climate based calculation reports
The output is a pair of numbers for each room. The first is the proportion of the reference plane meeting the target illuminance for at least half the daylight hours. The second is the proportion meeting a lower minimum target illuminance across almost the whole plane, which catches rooms with one bright zone at the window and a dark back half.
Both come from a grid of sensor points, an hourly climate file for a nearby location, and a geometric model of the room and everything outside it that blocks sky. The model is the part that goes wrong. A balcony above the window, a projecting fin, a deep reveal and the building opposite each remove sky, and all four are easy to leave out.
The standard expresses results against daylight hours rather than occupied hours, which matters when someone asks whether a bedroom result is meaningful at three in the afternoon. It is a measure of the daylight the room receives, not of how the occupier uses it.
Reference plane, grid and room setup
For dwellings the reference plane is normally taken at 0.85 metres above finished floor level, horizontal, covering the room area with a border excluded next to the walls. The grid should be fine enough that moving it would not change the answer, and a spacing of 0.5 metres or less is usual for domestic rooms.
Decide and record four things before any result is produced.
- Whether a combined living, kitchen and dining room is assessed as one space or as zones, and on what basis.
- Where the reference plane stops, which is usually the room boundary rather than the glass line.
- Whether built in furniture, islands and kitchen units are modelled.
- Which weather file location has been used and why it is appropriate for the site.
A combined room assessed as a single space will often pass where the kitchen zone alone would fail, and an authority that spots this after consultation will ask for the zonal result. Supplying both from the start removes the argument. The room naming should match the schedule of accommodation submitted with the application, because mismatched room names between the daylight appendix and the drawings cause more queries than the results do.
Target illuminance values by room type
The UK national annex gives lower target values for dwellings than the main body of the standard, recognising that homes are not offices. The values commonly applied in UK practice are these.
| Room type | Target illuminance | Area that must meet it | Time |
|---|---|---|---|
| Bedroom | 100 lux | At least 50 per cent of the reference plane | At least half the daylight hours |
| Living room | 150 lux | At least 50 per cent of the reference plane | At least half the daylight hours |
| Kitchen | 200 lux | At least 50 per cent of the reference plane | At least half the daylight hours |
Where a room serves more than one of these functions, the higher target applies to the zone that performs that function. A kitchen set at the back of a deep open plan flat is the hardest target in residential design and the one worth testing first, before the plan is fixed.
An illustrative result for a single aspect flat
The following is illustrative and uses rounded figures to show the method rather than to describe a real scheme.
A one bedroom flat on the second floor of a six storey block. The living, kitchen and dining room is 5.0 metres deep by 4.2 metres wide, 2.5 metres floor to ceiling. One window 2.4 metres wide by 1.5 metres high, head at 2.2 metres. A continuous balcony 1.8 metres deep sits directly above the window, and the facing building across a 22 metre street rises to a 25 degree obstruction angle measured from the window centre.
Assessed as a single space, the model returns the 150 lux living room target across roughly 44 per cent of the reference plane. Assessed zonally, the kitchen zone at the rear meets the 200 lux target across a little over a tenth of its area. The bedroom, shallower and with the same window proportion, clears the 100 lux target comfortably.
Three changes were tested against that baseline. Raising the window head from 2.2 to 2.4 metres produced the largest single improvement, because the top of the window sees the most sky. Reducing the balcony depth from 1.8 to 1.2 metres came second. Increasing glazing visible light transmittance from 0.68 to 0.76 produced the smallest gain of the three, which is the usual ranking and the reason specification changes rarely rescue a plan that is too deep.
Reflectance, transmittance and maintenance assumptions
Results move with the assumptions, so state them. Surface reflectances commonly adopted for domestic modelling are around 0.70 for ceilings, 0.50 for walls, 0.20 for floors and 0.20 for external ground. Use lower wall reflectance where a dark finish is specified, and say so.
Glazing visible light transmittance should come from the actual specification rather than a default. A clear double glazed unit sits higher than a solar control triple glazed unit, and the difference between 0.76 and 0.55 can be the difference between a pass and a refusal reason.
Maintenance and dirt factors, frame to glass ratios and any applied film all belong in the assumptions table. Where the facade specification is not yet fixed, model the worst credible case and say that is what you have done. An appendix that quietly assumes the best available glass and is then value engineered at tender has created a problem for whoever discharges the condition, which is the same discipline that applies to a materials schedule at discharge of condition stage.
Where the result diverges from vertical sky component
Vertical sky component is an external measure taken at the face of a window, used in BRE 209 to assess the effect of a proposal on existing neighbouring windows. Internal illuminance is a measure of the space behind the glass in the proposed scheme. They answer different questions and they can disagree.
A proposed flat can have perfectly acceptable vertical sky component at its windows and still fail internal targets, because the room is too deep, the ceiling too low or the balcony too generous. The reverse also happens: a shallow room with a large window can meet internal targets while the neighbouring property loses vertical sky component below the BRE guideline.
When an authority asks for a daylight and sunlight assessment, it usually wants both, covering effects on neighbours and amenity within the proposal. Reporting one and calling it the other is a common reason for a request for further information.
Reporting the inputs so the result can be checked
A reviewer should be able to rebuild the model from the appendix. That means a table of room areas and reference plane areas, the grid spacing, the reflectance and transmittance schedule, the weather file location, the software and version, the date of the model, and a plan showing the extent of surrounding massing included.
Show the failing rooms rather than reporting a scheme wide average. An average across 120 flats hides the twelve that cannot be fixed, and those twelve are what the planning committee will be asked about. Where some rooms fall short, set out what was tested, what the constraint is, and why the balance of the scheme is still acceptable in design terms. That argument belongs in the Design and Access Statement with the technical appendix behind it.
PlanScope drafts the supporting statements around a daylight appendix and flags where modelled inputs are still missing. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and daylight modelling is normally carried out by a specialist consultant.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Does climate based modelling replace the average daylight factor?
In current UK practice the illuminance methods in BS EN 17037 and its national annex have largely taken over from the average daylight factor approach for assessing daylight inside new homes. Some authorities still reference daylight factor in older supplementary guidance, so check the local policy position and report against whichever metric the development plan asks for.
What reference plane height should be used in a flat?
For dwellings a horizontal plane 0.85 metres above finished floor level is the usual convention, covering the room area with a border excluded next to the walls. The grid spacing should be fine enough that changing it would not alter the result, and 0.5 metres or less is standard for domestic rooms. Record both in the appendix.
Can a deep open plan flat ever meet the kitchen target?
It can, but the plan has to help. Raising the window head height, reducing balcony depth or projection, bringing the kitchen zone closer to the glazing and limiting room depth relative to ceiling height all move the result. Specification changes such as higher transmittance glazing usually produce the smallest improvement of the available options.
Should results be reported per flat or as a scheme average?
Per room, with the failing rooms identified. A scheme wide average conceals the small number of units that cannot be improved, and those units are what members and objectors will focus on. Present the full schedule, explain what was tested for the rooms that fall short, and set out the design reasoning in the Design and Access Statement.
Is a daylight assessment always required for a flat scheme?
Not always. Authorities typically ask for one where the proposal is in a dense urban context, where single aspect units are proposed, where balconies overhang habitable room windows, or where neighbouring windows could be affected. The local validation checklist and any adopted design guidance will say when it is expected. Check both before you submit.
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.