Average daylight factor and where it still applies
The 1, 1.5 and 2 per cent daylight factor targets came from a withdrawn standard, yet ADF still appears in adopted policies and prior approval assessments. What it measures and when to use it.
Average daylight factor is the ratio of internal illuminance to unobstructed external illuminance under a standard overcast sky, averaged across a room and expressed as a percentage. For years the average daylight factor planning test meant three numbers: 1 per cent for bedrooms, 1.5 per cent for living rooms and 2 per cent for kitchens. Those targets came from a British Standard that has since been withdrawn, and the guidance that carried them has moved on.
The metric has not disappeared. It survives in adopted local plan policies, in supplementary planning documents written before 2019, and in the natural light assessments that permitted development conversions still need. So architects have to know both what it measures and why newer work is done differently.
What the calculation actually does
The formula multiplies the net glazed area by the visible light transmittance of the glass and by the angle of visible sky measured from the centre of the window, then divides by the total area of all internal room surfaces and a factor accounting for internal reflectance.
Four things follow from that structure, and they explain most of the arguments about ADF results.
Glazed area is net, meaning the glass rather than the opening. Frames, transoms and mullions are deducted, and a heavy frame on a small window removes a surprising proportion of it.
The angle of visible sky is measured in section, from the centre of the window, to the top of whatever obstructs the view. A facing building, a deep balcony above or a reveal in a thick wall all cut it down. This is the term that punishes deep-plan and heavily balconied schemes.
The denominator is the total internal surface area, which means floor, ceiling and all walls including the window wall. Make a room bigger without making the window bigger and the result falls.
Reflectance is an area-weighted average of the internal surfaces, and a change from a dark scheme to a light one moves the answer materially. That sensitivity is the metric's main weakness.
The historic targets and where they came from
The 1, 1.5 and 2 per cent figures were minimum recommendations for dwellings where daylight is important to the occupants. They were never a legal standard, and they were always expressed as values below which supplementary electric lighting would normally be needed during daytime.
They also assumed no view of sky at all was the failure case, so a room achieving exactly 1 per cent was not being described as well daylit. It was being described as at the bottom of the acceptable range. Reading them as pass marks, which is how they are used in practice, overstates what they were meant to convey.
Kitchens carried the highest target because of the task lighting requirement. That has aged poorly now that open plan kitchen and living rooms are the norm, because the combined space is assessed against a single figure and the deepest part of the plan is a kitchen with no window.
Where the metric has been superseded
The British Standard that carried those targets was withdrawn and replaced by the European daylight standard, which assesses daylight provision through target illuminance achieved over a proportion of the reference plane for a proportion of daylight hours. That is a different question altogether: not an average ratio, but how often and over how much of the room a useful illuminance is reached.
The 2022 revision of the BRE guidance on site layout planning for daylight and sunlight aligned its interior recommendations with that approach. For new interiors the recommended assessment is now illuminance-based rather than ADF-based, which changes both the modelling effort and the way results are presented. The practical differences are set out in BRE 209 2022 changes for new interiors.
Two consequences for practice. Illuminance-based methods need climate data and a full model rather than a spreadsheet, so they cost more. And the results are harder to argue with, because they show distribution across the room rather than a single averaged number that can hide a dark rear third.
Where it still applies
Plenty of places, which is why the metric refuses to die.
- Local plan policies and supplementary planning documents that specify the ADF targets by name. Where a policy says 1.5 per cent, that policy is part of the development plan and section 38(6) of the Planning and Compulsory Purchase Act 2004 applies.
- Natural light assessments for prior approval conversions, where authorities often accept ADF as proportionate evidence of adequate natural light in habitable rooms.
- Quick comparative screening across many units early in design, where a full climate-based model is not yet justified.
- Refurbishment and conversion schemes where the existing condition was originally assessed on ADF and a like-for-like comparison is wanted.
- Rights of light and daylight loss work for existing neighbouring rooms, alongside the external obstruction tests.
The external tests are a separate family and are not replaced by any of this. Vertical sky component and the no-sky line still do their own job, as described in what vertical sky component measures.
The assumptions that move the answer
If you are going to present an ADF figure, present the inputs with it. Reviewers who have been given a bare percentage will assume the most favourable assumptions were used, and they are often right.
| Input | Typical issue | What to state |
|---|---|---|
| Glazing transmittance | Manufacturer's figure used without the maintenance factor | Product, transmittance value and any correction applied |
| Net glazed area | Frame deduction omitted | Window schedule with frame widths |
| Angle of visible sky | Balconies and reveals ignored | Section through each assessed room |
| Reflectance | Optimistic average assumed | Surface by surface reflectance and the weighted average |
| Room area | Open plan spaces split or merged inconsistently | Plan showing the assessed room boundary |
Maintenance factors deserve particular attention. Glass in a city centre at fourth floor level does not transmit what the datasheet says after two years, and applying a correction is the honest approach even though it lowers the result.
Offering it alongside newer metrics
On a scheme where the local policy names ADF but the guidance now points elsewhere, run both and report both.
Lead with the policy-compliant metric, because that is what the development plan requires and what the officer has to apply. Then present the illuminance-based results as supporting evidence, particularly where they show better performance than the averaged figure suggests. Where they show worse performance in part of a room, say so and explain the design response. Concealing it invites a consultant's review to find it.
One honest uncertainty worth naming in the report. The transition between the two approaches is not complete, local policies have not caught up in most authorities, and there is no settled convention for how the two sets of results should be weighed against each other where they disagree. Say which you regard as the better measure of the actual living conditions, and why, rather than presenting two numbers and leaving the officer to choose.
Alternative targets are also appropriate in dense urban contexts, where the standard figures were never intended to apply mechanically. That is covered in alternative daylight targets in dense urban areas.
A Daylight and Sunlight Assessment that states its method, its inputs and its limits is the one that survives a third party review.
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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 is average daylight factor?
It is the ratio of internal illuminance to unobstructed external illuminance under a standard overcast sky, averaged across a room and expressed as a percentage. The calculation uses net glazed area, glazing transmittance, the angle of visible sky from the window, the total area of internal surfaces and the average internal reflectance.
Are the 1, 1.5 and 2 per cent targets still valid?
They came from a British Standard that has been withdrawn, and the BRE guidance revised in 2022 points to illuminance-based assessment for new interiors. The figures remain relevant where an adopted local plan policy or supplementary planning document names them, because the development plan still applies.
Why did illuminance methods replace daylight factor?
Because an averaged ratio can hide a dark rear third of a room. Illuminance-based assessment asks how often and over how much of the reference plane a useful light level is achieved, using climate data for the location. It needs a full model rather than a spreadsheet, and the results are harder to argue with.
Where is average daylight factor still used?
In local policies and supplementary documents that name it, in natural light assessments for prior approval conversions, for rapid comparative screening across many units early in design, and in refurbishment schemes where the existing condition was originally assessed the same way and a like-for-like comparison is wanted.
Which assumptions most affect the result?
Internal surface reflectance, because an area-weighted average moves the answer materially; the angle of visible sky, which balconies and deep reveals cut down; the frame deduction from net glazed area; and the maintenance factor applied to glazing transmittance. State all of them, or a reviewer will assume the most favourable values were used.
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.