The Average Daylight Factor (ADF) was, for decades, the standard way to measure daylight inside new UK homes. If a planning officer, architect or daylight consultant talked about a room "achieving 2%", they meant ADF. But since the publication of BRE's BR 209 (2022) and the withdrawal of BS 8206-2, ADF is no longer the recommended metric for new daylight assessments — it has been replaced by illuminance-based targets from BS EN 17037.
Plenty of local plan policies, design guides and older officer reports still refer to ADF, so applicants regularly ask us what it means, whether it still applies, and what to use instead. This guide covers all three.
What is the Average Daylight Factor?
The Average Daylight Factor is the ratio of the average indoor illuminance on a working plane to the simultaneous outdoor illuminance under a standard overcast sky, expressed as a percentage. A room with an ADF of 2% receives, on average, 2% of the diffuse light available outside under a heavily overcast sky.
Because it uses an overcast sky, ADF deliberately ignores direct sunlight and orientation — it is a measure of diffuse daylight only. That made it simple and repeatable, which is exactly why it became the default test in BS 8206-2 and the 2011 edition of BRE's Site Layout Planning for Daylight and Sunlight (BR 209).
The ADF formula
ADF could be estimated with a single hand calculation:
ADF = (T × Aw × θ) / (A × (1 − R²))
- T — diffuse transmittance of the glazing (typically around 0.68 for clean double glazing, reduced for tints or external obstructions like balconies)
- Aw — net glazed area of the window (m²)
- θ (theta) — the angle of visible sky, in degrees, measured from the centre of the window in section
- A — total area of all room surfaces: walls, floor and ceiling (m²)
- R — area-weighted average reflectance of those surfaces (around 0.5 for typical light-coloured rooms)
The formula rewards bigger windows, more visible sky and lighter finishes, and penalises deep rooms and heavy external obstruction. It could be run from drawings in minutes — no software required — which kept assessment costs down for small schemes.
The old ADF targets (BS 8206-2 / BR 209 2011)
- Bedrooms: 1%
- Living rooms: 1.5%
- Kitchens: 2%
Where one room served several functions — the open-plan kitchen/living/dining room now standard in new flats — the highest applicable target was used, so most open-plan layouts were tested against 2%.
Why BR 209 (2022) dropped ADF
The 2022 edition of BRE's BR 209 aligned UK practice with the European daylight standard, BS EN 17037, which had already replaced BS 8206-2. The main criticisms of ADF were:
- It ignores climate and orientation. A north-facing room in Aberdeen and a south-facing room in Brighton could score identical ADFs while feeling very different to live in.
- A single average hides poor distribution. A deep room with one tall window can pass on average while the back half stays gloomy.
- It measures a ratio, not usable light. Occupants experience lux on a surface, not a percentage of an overcast sky.
BS EN 17037 instead sets target illuminance levels to be achieved across a fraction of the room's reference plane for at least half of the annual daylight hours. For UK dwellings, the commonly applied levels are 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens. We cover the mechanics in our guides to BS EN 17037 and the BR 209 (2022) daylight provision tests.
Daylight factors haven't disappeared — they've been recalibrated
Helpfully for anyone used to ADF, BR 209 (2022) allows the illuminance targets to be converted into equivalent median daylight factor targets that vary with location, because external daylight availability differs across the UK. In southern England the equivalent targets work out at roughly 0.7% for bedrooms, around 1% for living rooms and about 1.4% for kitchens, with slightly higher values needed further north. Two things changed from the old regime: the values are location-specific, and the target applies to the median point (at least half the working plane), not a simple room average — so distribution now matters.
Does ADF still apply to my planning application?
Sometimes, yes — in practice you may still meet ADF in three situations:
- Legacy policy wording. Some adopted local plans, SPDs and space standards still quote 1%/1.5%/2% ADF targets. Until those documents are updated, officers may ask how a scheme performs against them, even though the underlying British Standard has been withdrawn.
- Older permissions and discharge of conditions. Schemes consented before 2022 may have conditions written around ADF, and re-assessment should use a consistent metric.
- Prior approval and appeal history. Where earlier reports on a site used ADF, presenting both old and new metrics helps decision-makers compare like with like.
A competent daylight consultant will simply report both where needed: BS EN 17037 illuminance or median daylight factor results as the primary test, with ADF alongside for continuity. What you should not do is rely on ADF alone for a new application — the metrics measure different things, and a room that passed 2011-era ADF can fall short of the 2022 tests, particularly deep single-aspect rooms. See our guides to what changed in BR 209 (2022) and single-aspect flats for the common failure cases.
ADF vs VSC — don't confuse the two
ADF is often mixed up with the Vertical Sky Component (VSC), but they answer different questions. ADF (and its successors) measure daylight inside a proposed room — daylight provision for future occupants. VSC measures the sky visible at a window on an existing neighbouring building — the impact of your development on others. A full daylight and sunlight report for a contentious scheme usually needs both sides: provision within the scheme and effect on neighbours. Our guide to VSC, NSL and APSH explains the neighbour-facing metrics.
How Fortress Associates can help
If your scheme needs a daylight assessment — whether the council has quoted ADF, BS EN 17037 or BRE 2022 targets — our daylight and sunlight report service assesses your proposal against the current BR 209 (2022) methodology, with legacy metrics reported where a policy or condition requires them. Reports are typically delivered in 4–5 days with no advance payment, anywhere in the UK. Get in touch to discuss your site.
Sources & further reading
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