The 50/50 rule says a room is adequately lit in law when at least half its floor area still receives light from the sky, measured to a level called the 0.2% sky factor. Fall below that, and the loss starts to count as an actionable injury.
You will hear those two numbers thrown around by surveyors, letters and neighbours as if they settle everything. They do not. They are the starting grid for a rights of light analysis, not the finish line. If a development next door threatens your rooms, understanding what these figures actually measure tells you whether you have real leverage or a point that goes nowhere.
The key points:
- The 0.2% sky factor marks the dividing line between adequately and inadequately lit floor space.
- The 50/50 rule tests how much of a room falls the wrong side of that line.
- Passing or failing the numbers is where a claim begins, not where it ends.
What does the 0.2% sky factor actually measure?
The 0.2% sky factor measures how much of the visible sky a single point on a table or work surface can still see. Picture a spot roughly 850mm off the floor, about desk height. From that spot, how much open sky is left once the neighbour's new wall goes up?
The figure is a percentage of the whole sky dome. A point that can see 0.2% of the total available sky is treated in law as the minimum for that spot to count as well lit. It sounds tiny, and it is. That threshold traces back to the work of Percy Waldram in the early twentieth century, who reckoned that at around the 0.2% level a person with ordinary eyesight could just about read fine print without artificial light.
That is the honest origin of it. A rule of thumb about reading a newspaper, refined over decades into the number that now decides five and six-figure disputes. It is not perfect science. It is a workable convention the courts have accepted for a very long time.
How the 50/50 rule turns points into a verdict on the room
One point below the threshold does not lose you a room. The 50/50 rule looks at the whole floor.
A surveyor grids out the room and works out how much of the floor area sits above the 0.2% sky factor, meaning adequately lit, and how much sits below it. If half or more of the floor still reaches the threshold, the room is adequately lit in the eyes of the law, even if a corner has gone dark. If less than half reaches it, the room is inadequately lit.
That is the 50/50 rule in a sentence. Half the floor well lit is the accepted benchmark for adequacy.
The reason it matters is that light claims are not about the light you lose in the abstract. They are about whether what remains is enough. Colls v Home and Colonial Stores Ltd [1904] AC 179 set that principle over a century ago: you are entitled to enough light for the ordinary use of the room, not to every ray you once enjoyed. The 50/50 rule is the surveying tool that puts a number on "enough".
Adequate before, inadequate after: the two-picture comparison
The numbers only mean something as a before-and-after.
A rights of light analysis models the room twice. First as it stands today, with whatever sits opposite it now. Then again with the proposed development in place. The surveyor compares the well-lit area in each picture and measures how much has crossed from adequate to inadequate.
A room might sit comfortably above 50/50 today and drop well below it once the scheme is built. That shift is the injury. It is the gap between the two models, expressed as lost well-lit floor area, that drives whether you have a claim worth pursuing and roughly what it is worth. This is the same modelling that underpins a Waldram diagram, which plots exactly which parts of a room keep their light and which lose it.
Worth being clear here. This is a rights of light analysis, a private legal exercise about your easement. It is not a Daylight and Sunlight assessment, which is the separate BRE-based planning exercise a developer runs to support a planning application. The two use different methods and answer different questions, and the difference matters more than most people realise.
Why passing the numbers does not end the argument
Here is the part the letters rarely tell you. The 50/50 result is a threshold, not a verdict.
Fail the numbers and you have shown an actionable injury. That does not automatically hand you an injunction or a large payout. The court still asks whether the loss is more than trivial, how the parties have behaved, and what remedy fits. The Shelfer test, refined in Coventry v Lawrence [2014] UKSC 13, governs whether a court orders a building cut back or awards damages instead. The sky factor gets you through the door. What happens in the room is a separate question, and we walk through it in our piece on the Shelfer test and injunctions.
The reverse is also true, and this is where the honest broker part earns its keep. You can technically pass the 50/50 test and still have suffered a real, valuable loss, particularly in high-value rooms or where the light near a window has been badly hit. Surveyors have long argued that a crude floor-area count undersells losses in exactly the places people care about. A rigid pass on the numbers is not always the full story.
So a single figure quoted at you in a letter, whether it favours you or the developer, is rarely the whole picture. What matters is who modelled it, how carefully, and whether the analysis stands up when someone credible tests it. Evidence alone settles very little without the ability to enforce it, which is a point we make in more detail in why a survey on its own is not enough.
What the numbers mean for your position
If a scheme opposite you threatens rooms you actually use, the 50/50 test is the right first question to ask. Which rooms drop below the line, and by how much?
Get that modelled properly and you know where you stand. A clean fail across your main living rooms is a strong foundation. A marginal result in a box room you rarely enter is not. A good analysis tells you both, and tells you plainly which one you have.
Timing sharpens all of this. Your position is strongest before the development completes, because that is when a court has the widest range of remedies open to it, including ordering work stopped or cut back. Once a building is finished, the practical options narrow, even if the legal right is unchanged. Delay does not extinguish your claim, but it does quietly shrink what a court is likely to do about it.
The sensible move is to have the numbers modelled by someone whose job is to be right, not to win you as a customer. That means an independent rights of light analysis that tells you the honest result, then a clear read on whether that result gives you leverage worth acting on.
Common questions
Is the 0.2% sky factor written into law?
No. It is a long-standing surveying convention that the courts have accepted, not a statutory figure. No Act sets it down. It grew from Percy Waldram's early twentieth-century work and became the practical benchmark that experts and judges use, which is why it carries weight without being written into any statute.
Can I fail the 50/50 rule and still lose my claim?
Yes. Failing the numbers shows an actionable injury, but the court then decides on the remedy separately. Under Coventry v Lawrence [2014] UKSC 13, factors like the seriousness of the loss, delay and the conduct of both parties all shape whether you get an injunction, damages, or in a truly trivial case, nothing.
Does the 50/50 rule apply to every room?
The test is applied room by room to spaces that need natural light for their ordinary use. A living room, kitchen or bedroom counts. A cupboard, hallway or bathroom generally carries little or no protected expectation of light, so a loss there rarely supports a claim on its own.
If a development near you looks likely to push a room the wrong side of these thresholds, the useful step is to find out for certain. Start your claim assessment and we will tell you honestly whether the numbers give you a claim worth pursuing.
This article is general information about the law in England and Wales, not legal advice. Outcomes always depend on the facts.
Joshua Platt BSc (Hons) MRICS, Director, Daylight Protect
Daylight Protect is a rights of light claims specialist operating across England and Wales. We provide a fully funded solution that covers everything end to end, the surveyor, the solicitor, counsel and court fees, backed by legal insurance that covers the developer's costs if a claim does not succeed. There is no recovery, no fee, and no costs to you either way. We tell you plainly when you have a claim, and just as plainly when you do not.
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