Modelling assumptions in a daylight report, and why they matter

Two competent consultants can model the same window and reach different conclusions, and the reason is almost always the daylight modelling assumptions rather than the software. Room use, the baseline massing, surface reflectances, glazing transmittance and whether balconies were modelled will each move a vertical sky component result by more than the margin that separates compliance from harm. So the first thing to do with any daylight report, your own or the other side's, is to find the assumptions schedule and read it before the results.

The assumptions that move the numbers

Six inputs account for most of the variation between reports on the same site.

• What each neighbouring room is assumed to be used for.
• What the existing condition is taken to be, and whether unimplemented consents are included.
• The reflectance values applied to internal surfaces and to external obstructions.
• The visible light transmittance of the glazing and the frame factor deducted from the opening.
• Whether balconies, recesses, overhangs and window reveals are modelled or simplified away.
• Whether the assessment is reported against absolute targets, the relative 0.8 test, or both.

A report that does not state all six is incomplete, and a reviewer is entitled to say so in a consultation response.

Room use, and the guesswork behind it

Neighbour assessments are made without entering the neighbouring property. The modeller looks at the elevation, counts the windows, guesses which is a living room and which is a bedroom, and assigns a use.

That guess matters twice. It sets the target illuminance or average daylight factor for the room, and it sets the significance of any shortfall, because BRE guidance treats living rooms as more sensitive than bedrooms and kitchens as less sensitive than both.

Two habits improve the position. Record the basis for each assumption, whether that is the window pattern, an estate agent floor plan, a planning history drawing on the council's website or a knock on the door. And run the sensitive rooms both ways, so the report shows what happens if the assumption is wrong. An assessment that survives the pessimistic assumption is much harder to attack.

Baselines: existing, consented and notional

The result is a comparison, so the baseline decides the answer.

The usual baseline is the existing built condition. Where the site already has an implemented building, that building is removed for the proposed case and the comparison is straightforward. Where the site is cleared or the existing structure is small, the comparison can flatter the proposal, because any development at all reduces light from a very high base.

Some assessments therefore report a second scenario, based on a notional compliant envelope on the site, to show what a policy-compliant alternative would do. That is a useful sensitivity test rather than a substitute for the real comparison, and it should be labelled as such.

Unimplemented consents are the other trap. If a neighbouring site has an extant permission for a taller building, an assessment that ignores it may overstate the future baseline daylight enjoyed by the affected windows. Say what you have included and why.

Surface properties and glazing

BRE calculations use default reflectance values for internal surfaces and for external obstructions, and default assumptions about glazing and maintenance. Those defaults are reasonable for a generic case and wrong for many real ones.

State the values used. Where a room has been modelled with a lighter internal finish than a typical existing dwelling would have, the result is optimistic. Where an external obstruction has been given a light reflectance because the render is white, check whether it will stay white.

Glazing transmittance is the input most often left at a default. Secondary glazing, solar control coatings, heritage glass, and triple glazing all reduce it, and the frame factor deducted from the opening varies considerably between a slim aluminium system and a traditional timber sash with glazing bars. Take these from the specification, not the software library.

The details that get simplified away

Balconies are the classic omission. A balcony above a window cuts the visible sky substantially, and a model built from a massing block rather than the detailed elevation will not show it. Where the proposal has recessed balconies serving its own units, an internal daylight assessment that ignores them is not worth reading.

Deep reveals in masonry, brise soleil, projecting cills, overhanging eaves and external walkways all have the same effect at a smaller scale. So do the neighbouring building's own features, which is a reason to survey the affected elevation rather than model it from photogrammetry alone.

Storey heights assumed at a round figure across an existing terrace, when the actual heights vary, will move window centre positions by enough to matter at the margins.

Absolute targets against the relative test

BRE guidance offers both an absolute value and a proportional test, and the difference is often where the argument lives.

A window retaining at least 27 per cent vertical sky component is unlikely to be noticeably affected. Separately, a window retaining at least 0.8 times its former value is also unlikely to be noticeably affected. A window that drops from 40 per cent to 30 per cent fails the relative test while passing the absolute one, and a window that drops from 20 per cent to 17 per cent does the opposite.

Report both. An assessment that quietly presents only the test the proposal passes will be found out, and it damages the credibility of the parts of the report that were sound. The same discipline applies to annual probable sunlight hours and to the no-sky line.

Context also matters, and BRE says so. Its numerical guidance is advisory and intended to be applied flexibly, and in dense urban settings authorities frequently accept lower values where the pattern of development already produces them. That argument works when it is evidenced with a survey of existing values in the surrounding streets, and not when it is asserted.

How to interrogate a submitted report

When you receive a Daylight and Sunlight Assessment from another party, ask for five things.

1. The assumptions schedule, including room uses and the basis for each.
2. A window map with numbered windows keyed to the results tables.
3. The full results tables, not the summary of pass rates.
4. The baseline definition, including any consents included or excluded.
5. Confirmation of whether balconies, reveals and overhangs were modelled.

Then read the results for the worst affected windows rather than the headline percentage of compliant windows. A scheme reporting 92 per cent compliance can still contain a flat where the only living room window loses most of its light, and that flat is what the objection will be about.

Where amenity space rather than interiors is in issue, an Overshadowing Study on the 21 March sun path answers a different question and should not be conflated with the window results.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which amenity reports a scheme is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.