Daylight assessments at appeal: what inspectors test

Inspectors do not decide daylight appeals by comparing two sets of percentages and preferring the larger spreadsheet. They decide whether the living conditions of the affected rooms, or of the proposed rooms, would be acceptable in that location, and they use the numbers as evidence towards that judgement. A daylight assessment at planning appeal therefore wins or loses on three things: whether the model inputs are right, whether the comparison is honest, and whether the author can explain a numerical shortfall without pretending it does not exist.

That last point separates useful evidence from advocacy. An assessment that reports every result and then argues about weight is far harder to attack than one that has quietly chosen the flattering metric.

How competing assessments get resolved

Two consultants modelling the same window rarely produce the same vertical sky component. The gap almost always comes from inputs rather than from method: the assumed room depth behind an unsurveyed window, whether a boundary fence or an existing outbuilding was modelled, the treatment of balconies and overhangs, the reflectance values used for surrounding surfaces, and whether the existing building was modelled as built or as consented.

Inspectors handle that by asking where the divergence comes from and who can show their inputs. The party with a measured survey, an annotated model extract and a clear statement of assumptions tends to be preferred over the party with a glossier results table. Where neither side can resolve a room, the usual outcome is that the room is treated as uncertain and given less weight, which favours whoever does not need it.

So the practical lesson is dull. Survey the affected rooms if you can get access, record every refusal of access in writing, and set out assumptions as a numbered list rather than burying them in an appendix.

Where the guidance itself allows flexibility

The Building Research Establishment guide to site layout planning for daylight and sunlight sets out the familiar tests: the 27 per cent vertical sky component, the 0.8 former value ratio, daylight distribution against the no-sky line, and annual probable sunlight hours with its winter component. The guide states in terms that its figures are advisory, that they are not mandatory planning standards, and that they may be applied flexibly where a different target suits the context.

That sentence does a great deal of work at appeal, and both sides cite it. An appellant uses it to argue that a 0.7 ratio in a dense town centre is acceptable. A council uses it to argue that flexibility is a reason to look at the lived effect rather than a licence to ignore the guide. Inspectors generally accept both propositions and then ask what the context actually is.

The National Planning Policy Framework pushes in the same direction, asking authorities to take a flexible approach to daylight and sunlight where a development would otherwise provide acceptable living conditions. That provision arrived in the December 2024 edition and carries through the edition published on 17 August 2026, which uses policy codes in place of paragraph numbers, so cite the current code and edition rather than an older paragraph reference.

Alternative target values in an urban context

Where the guide's default targets are unrealistic, the defensible route is to set an alternative target and justify it from the surrounding townscape rather than from the scheme's needs. Two justifications work.

The first is an existing-conditions baseline. Model a sample of comparable windows in the same street or block, show what daylight they already receive, and propose a target consistent with that. An inspector can test this, because the sample is checkable.

The second is a policy-led target drawn from the adopted plan or a design code that sets its own figures, which some London boroughs and central area plans do. Where neither exists, saying "this is an urban area" without numbers behind it rarely persuades. And a target set only by reference to what the proposed scheme happens to achieve is circular, which inspectors say so about.

The weight a numerical failure carries

A breach of a guideline figure is a material consideration, and it is not determinative. Inspectors weigh how many rooms are affected, what those rooms are used for, how large the shortfall is, whether the room has another window, and what the existing condition already was.

A kitchen that loses light where the dining area retains a second window attracts less weight than a sole living room window. A bedroom loses less again, though the position is not uniform and depends on the policy wording. Rooms served by two windows are a recurring argument, and the arithmetic there is unintuitive, which we work through in our note on rooms served by two windows.

The other variable is the size of the shortfall. A ratio of 0.78 against a 0.8 guideline tends to be treated as compliance in substance. A ratio of 0.4 to a principal habitable room is a planning harm that needs a positive case to outweigh it, and in a scheme with no particular public benefit it will often be decisive.

Presenting results without special pleading

Report every room you modelled, including the ones that fail. Appeal evidence that presents only the passing rooms gets found out at the inquiry, usually by a question about a window visible on a submitted elevation that is missing from the results table.

Four habits make an assessment hold up. State the software and the version. Give the model extract as a figure, not only as a table. Separate existing, consented and proposed baselines, and say which one each ratio is measured against. Annotate the plans with room use, and say how room use was established, because an assumption that a room is a bedroom when the neighbour uses it as a study is the kind of detail that collapses a conclusion.

Where sunlight to gardens or amenity areas is in issue, an Overshadowing Study covering the equinox and the solstices is more use than a single March plot, and it closes off the argument that the worst case was never tested.

Cross-examination of daylight evidence

At inquiry, daylight witnesses get asked the same handful of questions. What did you survey and what did you assume. Why did you choose that baseline. Which rooms did you exclude and on what basis. What would the result be if the room depth were a metre greater. Would you accept that a ratio of X represents a noticeable change to the occupier.

Prepare the sensitivity answers in advance. Running the model again with the opposing side's assumptions, and reporting both, is more persuasive than defending a single run, and it removes the implication that the result depends on a choice you made. Non-residential sensitive uses follow different reasoning again, as our piece on daylight to hotel and care home bedrooms sets out.

Drafting the proof

Keep the proof short and put the method in an appendix. The body should say what the standards are, what the guide says about flexibility, what you modelled, what the results are, which rooms matter and why, and what the effect on living conditions would be in plain words. A planning witness then picks the conclusion up and weighs it.

One honest uncertainty is worth stating in the proof rather than conceding under questioning: where access was refused and room layouts were assumed, say so, say what you assumed, and say what the result would be on the alternative assumption. A Daylight and Sunlight Assessment that does this reads as a technical document rather than as a case, and inspectors treat it accordingly.

PlanScope drafts daylight and amenity supporting statements structured to UK planning policy, with gaps flagged where survey data is still needed. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and daylight modelling for appeal evidence should be prepared and signed by a suitably qualified author. Enter your postcode on PlanScope to see which reports your site is likely to need.