What Is a Waldram Diagram and How Is Daylight Modelling Used?

By Daniel Peddlestone · 10 June 2026

If you have ever looked into a rights of light claim, you have probably come across the term "Waldram diagram" and wondered what it actually means. It sounds technical, and it is, but the basic idea behind it is surprisingly straightforward. A Waldram diagram is the standard method used in England and Wales to measure whether a room receives adequate daylight. It has been the accepted approach in rights of light disputes for decades, and courts rely on it heavily when deciding whether a building has caused an actionable loss of light.

The method is named after Percy Waldram, an engineer who developed it in the early twentieth century. Waldram's insight was that not all parts of the sky contribute equally to the light inside a room. Light from directly overhead is far more intense than light from near the horizon. His diagram accounts for this by weighting different parts of the visible sky according to how much useful light they actually deliver to a point inside a room. The result is a visual map that shows how much "sky factor" reaches the working plane, which is typically measured at a height of 850mm above floor level, roughly desk height.

The key threshold in most rights of light cases is the so-called 50/50 rule. A room is generally considered adequately lit if at least half of its area on the working plane receives a sky factor of 0.2 percent or more. When a new building blocks enough sky to push more than half the room below that 0.2 percent threshold, the loss is likely to be treated as actionable. This does not mean you have no claim if the room was already below the threshold before the development. It means the analysis focuses on how much additional light you lose and whether that tips the balance.

To understand what a right of light is in practical terms, you really need to understand how it gets measured. Rights of light are not about sunshine or a view. They concern the diffuse daylight that enters through your windows, and Waldram analysis is how surveyors quantify that daylight in a way that courts accept.

Modern rights of light surveyors still use the Waldram method, but they now combine it with 3D computer modelling. Specialist software allows a surveyor to build a digital model of your property, the surrounding buildings, and the proposed development. The software then calculates sky factors across the working plane of each affected room, both before and after the new building goes up. This "before and after" comparison is central to every rights of light assessment. It shows exactly how much light you stand to lose, room by room, window by window.

The precision of this modelling matters enormously when it comes to how compensation is calculated. A developer or their surveyor will run their own daylight model, and your surveyor will run theirs. Differences in assumptions, such as the exact position of a window, the size of a room, or whether internal partitions are included, can produce very different results. This is why having your own independent surveyor matters so much. You should not simply accept a developer's assessment at face value.

It is also worth knowing that Waldram analysis applies specifically to rights of light under the Prescription Act 1832. It is not the same as the daylight and sunlight assessments that local planning authorities use when considering planning applications. Planning assessments typically follow the BRE guidelines, which use different metrics like Vertical Sky Component and Average Daylight Factor. A development might pass the planning daylight test but still cause an actionable interference with your right of light. These are two separate legal frameworks, and one does not override the other.

If you are unsure whether you can make a claim, the starting point is almost always a Waldram assessment of your property. A specialist surveyor will examine your windows, measure the rooms, and model the impact of the proposed or completed development. The results will tell you whether you have grounds for a claim and, if so, how strong that claim is likely to be.

You do not need to understand every detail of the mathematics behind a Waldram diagram. What matters is that you know it exists, that it is the accepted standard, and that it forms the backbone of virtually every rights of light dispute that reaches negotiation or court. If a developer's surveyor tells you that your loss of light is minimal, ask to see the Waldram analysis. If they have not done one, that should raise questions. And if you want to explore your options further, you can request a free assessment to find out where you stand.

Reference: Prescription Act 1832, s.3; Waldram method as accepted in Carr-Saunders v Dick McNeil Associates Ltd [1986] 1 WLR 922