What Is the 50/50 Rule in Rights of Light Claims?

By Stephen Buchanan · 4 August 2026

If you have heard the term "50/50 rule" mentioned in the context of a rights of light dispute, you are not alone. It is one of the most commonly searched concepts by property owners trying to understand whether a new building or extension has actually infringed their right of light. But the phrase itself can be misleading, so let's break down what it really means and how it works in practice.

The 50/50 rule is a shorthand used by surveyors and lawyers to describe the threshold at which a room is considered adequately lit by natural daylight. Under this principle, a room is said to receive sufficient light if at least half of its area, measured at table height (roughly 850mm above floor level), receives a minimum level of sky visibility. When a new development reduces the lit area so that less than 50% of the room meets this standard, the room is treated as inadequately lit, and an actionable infringement may exist. If you want to understand more about what a right of light is and how it arises, that is a good place to start before diving into the technical side.

This threshold comes from case law rather than statute. The leading authority is Colls v Home and Colonial Stores Ltd [1904], a House of Lords decision that established the test for what counts as adequate light. The court held that the question is not whether the claimant has lost some light, but whether the remaining light is sufficient for the comfortable use and enjoyment of the property according to ordinary standards. The 50/50 benchmark evolved from that judgment as a practical, measurable way for surveyors to apply the legal test.

Surveyors assess this using a technique called the Waldram method, which maps how much sky is visible from different points across a room. They plot contours showing where the light level falls below the accepted minimum, known as the "grumble line." If the area below this line exceeds half the room's floor area after the neighbouring development is built, that room crosses the threshold into inadequate light.

One thing that catches people off guard is that the 50/50 rule does not look at total light levels in a general sense. It focuses specifically on sky visibility, not sunlight, and it measures this at a working plane across the room. A room could feel darker overall without technically crossing the threshold, or it could cross it while still appearing reasonably bright on a sunny day. The method is about consistent, measurable daylight, not subjective impressions.

It is also worth knowing that different rooms within the same property can produce different results. A large, deep room with a single small window might already sit close to the 50% threshold before any neighbouring construction begins. That means even a modest new building could push it over the line. A smaller, well-windowed room might absorb a significant obstruction and still remain above the threshold. Surveyors assess each room individually, which is why a proper survey is essential if you want to understand whether you can make a claim.

The 50/50 rule matters enormously when it comes to remedies. Courts take a dim view, quite literally, of developments that push rooms below this threshold. If the infringement is clear and the developer proceeded despite knowing the risk, courts have the power to order demolition or modification of the offending structure, not just damages. That said, most disputes settle before reaching that stage. Understanding how compensation is calculated in these cases can help you set realistic expectations about what a claim might be worth.

Developers often commission their own rights of light assessments before building, using the same Waldram methodology. If they identify rooms that will be pushed below the 50% threshold, they may redesign their scheme, offer compensation upfront, or in some cases register a light obstruction notice under the Rights of Light Act 1959 to try to prevent claims arising. Knowing where you stand technically gives you leverage, whether you are negotiating a settlement or considering formal legal action.

If you think a new building near your property has reduced your daylight, the first step is getting a proper assessment from a specialist surveyor who can apply the 50/50 rule to your specific rooms. You can also explore practical guidance on next steps to understand the process from start to finish. The technical evidence almost always drives the outcome, so getting it right early makes all the difference.

Reference: Colls v Home and Colonial Stores Ltd [1904] AC 179 (House of Lords)