Average daylight factor against climate-based modelling
ADF gives one percentage per room under a uniform overcast sky. Climate-based modelling simulates every daylight hour of the year. Policy decides which you need.
Average daylight factor and climate-based daylight modelling answer different questions, and an argument about average daylight factor vs climate based methods usually turns out to be an argument about which question the local plan policy asked. ADF gives a single ratio of internal to external diffuse illuminance under a uniform overcast sky, with no reference to orientation, location or time of year. Climate-based daylight modelling simulates illuminance at points in a room hour by hour using a real weather file for the site, then reports how often a target level is met. One is a static geometric measure. The other is an annual performance measure.
Both can be the right answer. Which one your council will accept depends on the policy wording and on what the scheme is being criticised for.
What each method calculates
ADF is derived from the window area, the visible light transmittance of the glazing, the angle of visible sky at the window, the total internal surface area and the area-weighted internal reflectance. Because the sky is treated as uniformly overcast, a north-facing and a south-facing window of identical geometry return the same number. The familiar recommended minima of 2 per cent for a kitchen, 1.5 per cent for a living room and 1 per cent for a bedroom come from the former British Standard on daylighting in buildings, and they have been written into a great many adopted local plans.
Climate-based daylight modelling takes a geometric model with assigned reflectances and transmittances, applies an hourly climate file, and reports illuminance across a reference plane for every daylight hour of the year. The current European and British standard on daylight in buildings sets its recommendations in those terms: a target illuminance achieved over a defined fraction of the reference plane for at least half the daylight hours, with a lower minimum target achieved over almost all of the plane. The UK national annex gives the target values used for residential rooms.
The second method knows the difference between a north-facing single aspect flat and a south-facing one. The first does not, which is exactly why it survives in policy: it is quick to check and hard to game.
Inputs each method needs
ADF needs the room geometry, the window schedule, the glazing specification, a value for the angle of visible sky, and reflectance assumptions. It can be done in a spreadsheet, and a reviewer can repeat it in twenty minutes.
Climate-based modelling needs a three dimensional model of the room and everything that obstructs it, surface reflectances for walls, floor, ceiling and external surfaces, glazing transmittance and any shading or frame geometry, a climate file appropriate to the site, a defined reference plane height and grid spacing, and a stated set of daylight hours. Each of those is a choice, and each choice moves the result.
That is the practical difference. ADF has few enough inputs that two consultants will usually agree. Climate-based modelling has enough that two consultants will not, unless the method statement pins the assumptions down. Pin them down: grid spacing, plane height, reflectances, weather file, hours assumed, treatment of curtains and balconies.
Where ADF still appears in policy
Plenty of adopted local plans and residential design guides still name ADF and its percentage targets. Where that is the policy, the assessment has to report ADF, because an officer applying the development plan cannot disregard the figure it specifies and a committee report will quote it.
What you can do is report ADF against the policy and add the illuminance-based results as supporting evidence. That is the reporting pattern most councils accept, and it lets you deal honestly with the case where a room passes one test and fails the other. The way local plan amenity policies use BRE 209 is worth checking in parallel, because the same plan often mixes a 2022 guidance reference with a target drawn from a withdrawn standard.
Where the question is the effect of your scheme on existing neighbouring windows, neither method is the primary tool. That is the vertical sky component, no sky line and annual probable sunlight hours territory of the building research guidance, and a Daylight and Sunlight Assessment usually has to cover both the neighbour tests and the internal performance of the new accommodation.
Target illuminance and exposure fractions
The illuminance-based approach reports two things rather than one. A target illuminance achieved over a stated fraction of the reference plane, and a minimum target illuminance achieved over a larger fraction. Both have to be met for the recommendation to be satisfied, and reporting only the first is a common error that a careful reviewer will catch.
It also produces results that feel counter-intuitive at first. A deep room with a large window can meet the target near the glazing and fail the minimum across the back half. A shallow room with a modest window can pass both. The spatial distribution is the information, and the single headline percentage throws it away. Plot it.
| Aspect | Average daylight factor | Climate-based daylight modelling |
|---|---|---|
| Sky model | Uniform overcast, standard | Hourly real weather file for the location |
| Orientation sensitive | No | Yes |
| Output | Single percentage per room | Illuminance across a grid for every daylight hour |
| Source | Former British Standard on daylighting, carried into local plans | Current European and British standard on daylight in buildings |
| Typical effort | Spreadsheet or simple model | Full three dimensional model and simulation |
| Reproducible by a reviewer | Readily | Only if assumptions are documented |
Which method a council will accept
Ask before you model. The answer tends to fall into one of three patterns. Authorities whose plan names ADF want ADF and will treat anything else as supplementary. Authorities working to the current standard, including many London boroughs applying the Mayor's housing design guidance, want the illuminance-based results. A third group has no stated preference and will accept either provided the method is explained.
Where the application is contentious, assume an objector will commission a review. That review will attack whichever method you used by pointing at the other, so cover both and explain the difference in two paragraphs rather than leaving the gap open. Where the scheme contains single aspect flats with daylight problems, the illuminance-based results are usually the ones that show whether a design change has actually worked.
Illustrative example
A third-floor north-facing living and dining room, 3.6 metres wide by 6.2 metres deep, with a single window of 2.6 m² of glazing, sits behind a 12 metre obstruction at 18 metres. The ADF calculation returns a figure above the 1.5 per cent living room target, so the room passes the local plan test. The climate-based model shows the target illuminance met over roughly the front third of the reference plane and the minimum target failing across the rear two metres. Both results are correct, and together they tell the designer to shorten the room or add a second opening rather than to argue about methods. This example is illustrative and is not taken from a real assessment.
Reporting both without contradiction
Write the method section first and the results second. State for each room which tests were run, on what geometry, with which assumptions, and where the two methods disagree say so in the text rather than leaving the reader to notice.
Three habits keep an assessment defensible:
- Give every room a single identifier used consistently on drawings, in tables and in the model.
- Record the drawing revision the model was built from, because a late plan change invalidates the results silently.
- State the reflectances and the weather file in the report body, not in an appendix nobody opens.
One honest uncertainty: the relationship between the targets in the current standard and the percentage figures in older adopted policies has not been settled by any national guidance, so a room can comply with one and not the other with no authoritative tie-breaker. Say which you have applied and why, and expect the point to be argued if the application goes to appeal.
Daylight modelling is specialist work, and councils expect a suitably qualified author. Browse the PlanScope report types to see where a daylight assessment sits in the wider document set for a residential scheme.
This is general guidance, not legal advice, and daylight assessment should be carried out by a suitably qualified professional. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is average daylight factor still a valid method?
It remains valid where an adopted local plan or design guide specifies it, and many do. The percentage targets of 2 per cent for kitchens, 1.5 per cent for living rooms and 1 per cent for bedrooms came from the former British Standard on daylighting. An officer applying the development plan cannot ignore a figure the plan names.
What does climate-based daylight modelling actually report?
Illuminance across a reference plane for every daylight hour of the year, using a weather file for the site. The current standard frames its recommendation as a target illuminance achieved over a stated fraction of the plane for at least half the daylight hours, plus a lower minimum target achieved over almost all of it. Both parts have to be met.
Why do the two methods give different answers?
ADF assumes a uniform overcast sky, so orientation and location make no difference to the result. Climate-based modelling uses real hourly data, so a north-facing room and a south-facing room with identical geometry return different results. A deep room can pass ADF while failing the minimum illuminance target across its rear section.
Which method should I submit?
Ask the authority before modelling. Where the local plan names ADF, report ADF against the policy and add illuminance-based results as supporting evidence. Where the authority works to the current standard, lead with the illuminance results. On a contentious scheme, cover both, because an objector's reviewer will attack whichever one you left out.
Do these methods assess the effect on my neighbours?
No. Effect on existing windows is assessed using the vertical sky component, no sky line and annual probable sunlight hours tests in the building research guidance on site layout planning for daylight and sunlight. A daylight and sunlight assessment usually has to cover the neighbour tests and the internal performance of the new rooms separately.
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.