The 0.8 rule for retained daylight and how it works

The 0.8 rule is the proportional half of the BRE daylight test, and it is the half that decides most urban schemes. A window fails on daylight only when its vertical sky component falls below the numerical target and below 0.8 times its former value. Meet either limb and the guidance does not identify a noticeable reduction. That conjunction is what keeps dense city schemes viable, and misreading it is the commonest error in objection letters.

The same 0.8 ratio recurs across the other measures in BRE 209, which is why it is worth understanding as a principle rather than as a single number attached to one test.

Why a proportional test sits alongside an absolute one

An absolute target answers the question of whether a room receives enough daylight. A proportional test answers a different question: whether the occupant will notice the change.

Those come apart in two directions. A window with generous existing daylight can lose a third of it and still be comfortably above the numerical target, and nobody will complain. A window already below target can lose a sliver and, because the baseline was small, register a large percentage loss without anyone perceiving it.

BRE 209 handles that by requiring both limbs before an adverse effect is identified. The wording matters: the reduction is noticeable where the value is both below the target and below 0.8 of the former value.

Which means a single-limb argument is incomplete. An objection that cites a 40 per cent reduction without stating the resulting absolute value has said nothing about whether the room is adequately lit, and an applicant's assessment that cites a healthy absolute figure without the ratio has said nothing about the change.

How the comparison is made

The former value is the value in the existing situation, measured on the same basis as the proposed value, at the same reference point on the outside face of the window.

Pin down the baseline before anything else. The existing site as built is the usual starting point, but there are cases where a different baseline is more informative: a site with an extant permission, a site cleared of a building that has already been demolished, or a phased scheme where phase one is consented but unbuilt. Assessments that test against more than one baseline and present them side by side are far more useful to a planning officer than assessments that assert a single one.

Then the arithmetic. Divide the proposed value by the former value. A ratio of 0.80 or above passes the proportional limb. Below that, the window fails the limb, and whether it fails overall depends on the absolute value.

Present the numbers as a table, one row per window, with the room use named. Existing value, proposed value, ratio, pass or fail on each limb, and the conclusion. Anything less makes the officer do the arithmetic themselves, which is how disagreements start.

Check the current edition for the figures themselves. BRE 209 was revised and the detail moved, so quoting a number remembered from an older assessment is a way to lose an argument at appeal.

Where existing values are already low

This is the situation the rule handles worst, and the guidance acknowledges it.

Take a ground floor window under a deep balcony in a city centre. Its existing vertical sky component might be in single figures. A new building opposite reduces it by a couple of points, which is a loss of nearly a third in proportional terms and close to nothing in perceptual terms. The 0.8 test fails dramatically while the actual change is hard to detect.

The reverse case exists too. A window with a generous existing value can pass the proportional limb while losing more daylight in absolute terms than the balcony window ever had.

BRE 209 is explicit that its numerical guidance is not mandatory and should be applied flexibly, with judgement about the context. Inspectors have accepted that reasoning repeatedly in dense urban settings, and the London Plan's design policies anticipate it for tall and dense development. The practical response is to report the ratio honestly, then explain in words what the change means for the room, with the absolute values alongside.

Where proportional loss understates real harm

Flexibility cuts both ways, and this is the part applicants gloss over.

A suburban living room with good existing daylight can pass the 0.8 test comfortably and still lose its outlook entirely to a flank wall 4m away. Vertical sky component is measured at a point on the window plane. It says nothing about what is visible through the window, nothing about the sense of enclosure, and nothing about whether the room now looks at brickwork.

Daylight distribution catches part of that, because it measures how far into the room direct skylight reaches. Where the no-sky line moves substantially towards the window, the back of the room goes dark even though the window plane still sees sky.

So report both measures for habitable rooms, not just the vertical sky component. A Daylight and Sunlight Assessment that gives vertical sky component alone is doing half the job, and officers increasingly say so.

Common errors in the calculation

Five recur often enough to check for on every report.

1. Reference point taken at the centre of the glazing rather than at the window on the outside face of the wall, which shifts results where reveals are deep.
2. Room uses assumed rather than established, so a bathroom is assessed as a bedroom or a kitchen-diner is treated as a non-habitable kitchen.
3. Existing site modelled from planning drawings rather than survey, with the neighbouring building's actual window positions guessed.
4. Balconies, overhangs and reveals omitted from the model, which flatters the existing case and therefore exaggerates the proportional loss.
5. The ratio reported to two decimal places against values that are themselves approximations, implying a precision the method does not have.

Where the affected property cannot be entered, the room layout has to be inferred, and there is an established way of doing it. The approach to assessing daylight to a property you cannot access sets out the assumptions to declare.

Presenting both tests together

The report that persuades does three things. It states the baseline and why it was chosen. It gives a table with both limbs and the conclusion per window. And it explains, in plain sentences, which windows are materially affected and what the applicant has done about them.

Where the scheme affects a school or nursery, different reference conditions apply to teaching spaces, and the daylight targets for schools and nurseries are worth checking before modelling begins. Where the issue is amenity space rather than internal rooms, an Overshadowing Study answers a question the vertical sky component cannot.

Then say what changed in the design as a result. A massing set back at the upper floors, a chamfer to a corner, a reduced depth to a rear wing: named changes with the resulting numbers carry more weight than any amount of argument about flexibility.

Enter your postcode on PlanScope to see which amenity and daylight documents your scheme 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.