Modelled daylight results compared with hand calculation

The choice between daylight modelling versus hand calculation is not really a choice about accuracy. It is a choice about what the result has to do. Vertical sky component and no sky line calculated by the geometric methods in BRE 209 produce numbers a case officer can check against your own drawings. Climate based daylight modelling produces illuminance results that describe the rooms far better and that nobody outside the modelling team can verify without the model file. On most schemes the answer is to use both, each for the question it answers.

Reviewability, not precision, is what decides which number carries weight in a planning decision.

What each method actually produces

The geometric methods assess availability of sky, and they do it from a fixed reference point. Vertical sky component measures the proportion of unobstructed sky visible at the centre of a window on the outside face, with 27 per cent as the reference value in BRE 209. No sky line, or daylight distribution, maps the part of a working plane within a room that can see the sky, and the test compares the area before and after the proposed obstruction. Annual probable sunlight hours does the equivalent for sun, with the familiar 25 per cent annual and 5 per cent winter figures.

Climate based daylight modelling asks a different question. It calculates illuminance on a grid inside the room for every hour of a typical year, using local climate data, and reports how often and over how much of the room a target illuminance is met. The targets come from BS EN 17037 and its UK national annex, which gives recommended levels of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, met over half the reference plane for half the daylight hours.

One tells you what the sky looks like from the window. The other tells you how bright the room is in April at four in the afternoon. They are not competing estimates of the same quantity.

Inputs the software needs that a hand method does not

A vertical sky component calculation needs the window position and the geometry of the obstruction. That is all. You can do it from a section and a site plan, and a competent reviewer can repeat it.

A climate based model needs considerably more, and every additional input is a place where the result can be shaped.

1. Internal surface reflectances for walls, floors and ceilings, which are design assumptions rather than measurements.
2. Glazing visible light transmittance for the specified unit, which changes with coatings and frame type.
3. A maintenance or dirt factor.
4. Room layouts and reference plane heights, which on a neighbouring property are usually assumed because you cannot survey inside someone else's flat.
5. Context geometry including surrounding buildings, boundary treatments and ground reflectance.
6. A climate file for the nearest representative location.

None of those is unreasonable. All of them are choices. Raising assumed wall reflectance from 0.5 to 0.7 across a scheme improves results everywhere, and no reader of the PDF would know it had happened unless the report states the value.

Where the two methods diverge

The classic divergence is the deep room with a large window. Vertical sky component at the window head can fail while illuminance across the front half of the room is comfortable, because the metric ignores window size entirely. A tall narrow slot and a wide picture window with the same sky view score identically on vertical sky component and very differently on any illuminance measure.

The reverse happens too. A room with generous glazing facing a light-coloured courtyard can pass illuminance targets while the occupant has almost no view of sky, which is the condition vertical sky component was designed to detect.

Our note on daylight to a single storey rear extension on a bungalow shows the geometric case at its most useful, where the 45 degree and 25 degree tests do the work and nobody needs a model. And where a neighbour's room is lit from more than one opening, the geometric answer gets slippery in a way our piece on a neighbour's room served by two windows sets out.

Transparency and the reviewer's ability to check

A planning officer, a neighbour's agent and an inspector all have the same problem with a modelled result. They can read the output, and they cannot test it.

So the report has to carry its own audit trail. A modelled daylight and sunlight assessment should state the software and version, the climate file used, every reflectance and transmittance value, the assumed room layouts with a note of which are surveyed and which are inferred, the reference plane height, and the grid spacing. Where neighbouring layouts are assumed, the report should say what was assumed and why, and ideally test a sensitivity case.

Two reporting habits should raise an eyebrow. Results presented only as a pass rate across the whole scheme, with no room-by-room schedule, hides the worst units. And a change in method between the submitted assessment and the response to objections almost always means the first method gave an answer the applicant did not like.

Geometric results are not immune. Measuring vertical sky component from the wrong reference point, or taking the no sky line on a plane at the wrong height, produces numbers that look authoritative and are simply wrong. The difference is that a reviewer with the drawings can find the error.

Which method councils accept

BRE 209 remains the reference point in the overwhelming majority of English planning decisions, and its geometric tests are what most development plan policies and most committee reports talk about. The 2022 third edition moved the assessment of new accommodation towards the illuminance approach in BS EN 17037 while keeping the geometric tests for effects on existing buildings, which is a sensible split and the one worth adopting.

In London, the Greater London Authority's housing design guidance points towards BS EN 17037 methods for internal daylight in new homes, alongside London Plan policy on housing quality. Verify the current guidance, since it has been updated more than once.

Outside London, assume the officer will want the geometric numbers whatever else you provide. A report that leads with modelled illuminance and relegates vertical sky component to an appendix invites the question of what the appendix shows.

Presenting both where the result is contested

Where a scheme has a real daylight problem, the sequence that works is: geometric tests for every affected neighbouring window, a clear table of the ones that fall below the reference values, and then a modelled analysis of those specific rooms to explain what the shortfall means in practice. The modelling is an explanation of the geometric result, not a replacement for it.

Pair that with the context. A vertical sky component of 20 per cent in a dense urban street is a different proposition from the same figure in a suburban cul-de-sac, and BRE 209 says so itself. Where the surrounding townscape is the reason for the result, an overshadowing study covering amenity space and a short statement on the existing character give the officer a basis for the planning balance.

Choosing a method at the start of a project

Fix the method before the massing is fixed, because retrofitting a daylight strategy to a consented envelope is the most expensive way to do this.

Agree three things at pre-application stage. Which metrics the authority expects for effects on neighbours, which for internal amenity in the new units, and whether surveyed internal layouts of neighbouring properties are expected or whether assumptions will be accepted. Then build the geometry from a measured survey rather than from the planning drawings, since the gap between the two is frequently larger than the gap between the two assessment methods.

Enter your postcode on PlanScope to see which amenity and daylight documents a scheme is likely to need before you commission the modelling.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A daylight and sunlight assessment is normally expected from a suitably qualified author, and PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.