Reviewing somebody else's daylight report

Reviewing a daylight and sunlight report submitted by somebody else is a forensic exercise, and the answers are almost always in the appendices rather than in the summary. An optimistic assessment rarely contains a false number. It contains a model that omits three windows, a room labelled as a bedroom when it is a living room, a baseline taken from a consented scheme rather than the building that exists, and a relief applied to the results that fail but not to those that pass.

The reviewer's job is to find which of those applies, say what it does to the conclusion, and tell the officer what to ask for.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Reading the summary against the appendices

Start with arithmetic. Count the windows in the results tables and compare the total against the number visible on the existing elevations of the affected properties. A discrepancy is the single most productive first finding.

Then compare the summary narrative against the data. Summaries routinely report that a high proportion of windows meet the guidance, which is true and incomplete if the failures are concentrated in one property or on one elevation. Twelve failures spread across two hundred windows reads differently from twelve failures in a single block of flats where every affected flat has one room.

Three checks that take fifteen minutes and usually find something:

• Whether the stated number of assessed windows matches the number of windows on the drawings.
• Whether properties identified in the planning history or in objections appear in the results at all.
• Whether any result tables are missing for a metric the methodology said would be assessed.

Where the assessment says a metric was not required, look for the reason. A no-sky line assessment omitted "because internal layouts were unavailable" is a gap the applicant could usually have filled by asking.

Checking the model geometry against the drawings

The model is where optimism hides, and the three dimensional geometry is rarely published in a form that can be checked directly. What can be checked is the description.

Compare the stated storey heights, window positions and window dimensions against the existing survey drawings for the neighbouring properties. Check whether the proposed building was modelled to the heights on the submitted elevations, including any plant enclosure or parapet.

Reveals matter. A window modelled flush with the façade admits more light than the same window set back 300mm in a reveal, and modelling existing windows flush while modelling proposed ones recessed is a quiet way to improve a comparison. Ask whether reveal depths were modelled and how they were established.

Balconies and overhangs matter more. An existing balcony above a window reduces its sky view considerably, and omitting balconies from the existing model understates the baseline, which makes the proposed reduction look smaller than it is. Where the neighbouring building has projecting balconies, their treatment should be stated explicitly.

The question to put to the applicant is simple and hard to deflect: provide a view from each assessed window position showing the modelled obstruction, so the geometry can be compared with a photograph taken from the same point. The note on daylight model accuracy, survey, massing and reveals sets out what a defensible model records.

Which windows were omitted and why

Windows get excluded for three stated reasons, and each needs testing.

Non-habitable rooms are properly excluded, since the guidance concerns rooms where daylight is required. But the exclusion depends on knowing the room use, and assessments frequently assert it from external appearance. A small window at half-landing height is probably a stair. A full-height window on the first floor of a terrace is probably not a bathroom just because it has obscure glazing.

Windows at an oblique angle to the development are sometimes excluded as unaffected. That is a judgement, not a rule, and it should be supported by a stated angle or distance criterion applied consistently.

Windows in buildings with a live planning consent are sometimes assessed against the consented scheme rather than the building as it stands. That is a legitimate approach in some circumstances and a significant choice in all of them, and it needs to be flagged rather than buried in a methodology paragraph.

Ask for the room use evidence. Internal inspection, letting particulars, historic planning drawings and council tax records are all available, and an applicant who assessed room use from the pavement should say so.

Room use assumptions and their effect

Room use drives the target. The guidance treats living rooms, dining rooms and kitchens as more sensitive to daylight loss than bedrooms, so recategorising a room changes whether a result is reported as a breach.

In flats this becomes decisive. A combined living, kitchen and dining space served by one window is a different proposition from a bedroom served by the same window, and in older conversions the room uses are not obvious from outside. An assessment that labels every rear room in a block as a bedroom has made an assumption worth testing.

For sunlight, the orientation screening matters as much. Annual probable sunlight hours are assessed for windows within 90 degrees of due south, so a window reported as not requiring assessment should have its orientation checked against the site plan rather than taken on trust.

Where the development itself includes new homes, apply the same scrutiny to the internal daylight results for the proposed units. Schemes that carefully assess the neighbours and skate over their own single-aspect north-facing flats are common, and the internal result is a material consideration in its own right.

Selective use of the relative reduction allowance

The guidance allows that where a result falls below the recommended value, the effect may still be acceptable if the reduction relative to the former value is small, with a proportional threshold applied to the before and after figures.

Used properly this is sensible, because a window that was already heavily obstructed cannot lose much more in absolute terms. Used selectively it is a way of reporting failures as compliance.

What to look for:

1. Whether the relative test is applied consistently to all results, or only to those that fail the absolute value.
2. Whether both the absolute figures and the ratio are reported, or only the ratio.
3. Whether a window with a very low existing value is described as compliant because its proportional loss is small, without discussing that the absolute level is poor in both states.
4. Whether the same approach is taken for daylight and for sunlight results.

Point three is the honest difficulty rather than a trick. A window at a very low existing value really cannot lose much, and the guidance recognises it. The review should say that the proportional test is satisfied and that the resulting condition is poor, rather than pretending either half of that is the whole answer. The note on daylight assessment where the baseline is already poor deals with this directly.

Comparing against a no scheme baseline

Ask what the existing condition was taken to be. There are three candidates and they give different answers: the building as it physically stands, the site cleared, and the site with a consented but unbuilt scheme on it.

The second of these appears more often than it should. Modelling the baseline as a cleared site inflates the existing values and therefore the apparent loss, which is unusual in an applicant's report but common in an objector's. Modelling the baseline as a consented scheme reduces the apparent loss, which is the applicant's version.

Neither is automatically wrong. Both need stating, justifying and, where there is any doubt, running as an alternative so the officer can see both results. A reviewer who identifies an undisclosed baseline assumption has found the most consequential issue available.

Writing a review an officer can act on

Officers need findings they can convert into a consultation request or a committee paragraph. Structure the review accordingly.

Lead with a short statement of what the assessment concludes and what the review finds, in plain figures. Then take each issue in turn: state the specific point, explain what it does to the result, and say what should be submitted to resolve it. Avoid adjectives. A sentence saying the assessment omits eleven windows on the rear elevation of a named property is worth more than a paragraph describing the report as inadequate.

End with a request list rather than a demand for refusal. Three or four specific items, each capable of being produced in a fortnight, is a document an officer can send to the applicant. A review that concludes only that the scheme is unacceptable gives them nowhere to go.

Where the matter reaches appeal, the weighting Inspectors give to breaches is its own subject, covered in how inspectors weigh daylight breaches at appeal. The underlying methodology sits in the Daylight and Sunlight Assessment and, where sun on the ground rather than at windows is the issue, the Overshadowing Study.

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