Tall building daylight assessment: scoping the study area

On a tall building, scoping is the assessment. Once the receptor list is fixed, the modelling is mechanical and the results are what they are. Get the list wrong and you buy a study that a single question at committee can undo.

The mistake runs both ways. Too narrow, and an omitted property becomes the story. Too wide, and you spend the fee modelling flats that were never going to be affected, then have to summarise 900 windows in a way a planning committee can follow.

Setting the radius and selecting properties

Start with geometry rather than a number. A fixed radius, whether 100 metres or 250, has no basis in the guidance and is easy to attack.

The defensible approach uses the screening test in BRE guidance. Draw a section from the centre of the lowest relevant window on each candidate building to the top of the proposed development. Where that line subtends less than 25 degrees to the horizontal, a material reduction in diffuse daylight is unlikely and detailed calculation is not usually needed. Everything that fails the test comes into the study.

In practice that produces a study area shaped by the site rather than a circle. It reaches further to the north of the building, where shadows fall, and further along open streets and across low-rise blocks than it does behind an existing tall neighbour.

Three things to capture as you build the list:

• Every residential window serving a habitable room, including flats above commercial units, which are easy to miss from street level.
• Non-domestic receptors where daylight is expected for the activity, such as schools, nurseries and healthcare buildings.
• External amenity space: gardens, courtyards, roof terraces, playgrounds and any public open space the council's policies protect.

Screening obviously unaffected receptors

Screening is a positive act, not an omission, and it should be written up as such.

For each property excluded, record the address, the reason, and the geometric evidence. A short table with the subtended angle for each screened building, cross-referenced to a plan and one or two representative sections, does the job and takes an afternoon.

That record is what protects the assessment later. When an objector asks why their block was left out, the answer is a measured angle rather than a professional opinion, and the officer can put the point to rest in the report instead of deferring the application.

Do not screen out a property because it is in poor condition, because it is due for redevelopment, or because it already has low daylight. Existing conditions belong in the analysis, not in the screening.

The other trap is relying on published mapping for window positions. Building outlines from open data give you footprints and approximate heights, and nothing about where the habitable rooms are. A street-level survey, or at minimum a photographic record from the public highway with storey heights checked against a measured survey, is what turns a footprint into a receptor.

Transient overshadowing and sun path studies

Window metrics say nothing about a garden at four o'clock. For amenity space, the relevant work is sun hours on the ground and shadow path studies.

The convention is to model the equinox, 21 March, as the benchmark date, with 21 June and 21 December as the summer and winter extremes, showing shadows hourly through the day. BRE guidance uses a two-hour sunlight test on 21 March for amenity areas, applied to a proportion of the space, with a proportional test where the existing condition already falls short.

On a tall building, the transient study often matters more than the window results. A slender tower may cause a small, brief shadow that sweeps across a park, while a squat block of half the height parks a permanent shadow on the same space. The numbers do not show that. The animation does.

Illustrative example: a 22-storey residential tower north-west of a public square. Window-based results for the surrounding blocks are comfortable, because the tower sits across a wide road. The shadow study shows the square losing late-afternoon sun in March across two-thirds of its area, which becomes the main issue at committee. Better to know at scoping than at the meeting.

Agreeing scope with the council in advance

Put the receptor list in a pre-application submission and ask for written confirmation. The questions worth settling:

1. Which properties are in the study, and which are screened out.
2. Whether the baseline is existing buildings only, or existing plus consented schemes.
3. Whether a notional or mirror-image baseline is to be used where an adjoining site is undeveloped.
4. Which amenity spaces are assessed, and to what test.
5. Whether the council intends to appoint an independent reviewer, and at whose cost.
6. What format the results should take, since some authorities have a preferred reporting template.

Getting answers to those six takes one meeting and one letter. Not getting them costs a deferral.

Reporting a large receptor set clearly

A tall building assessment can run to hundreds of windows, and the reporting challenge is real. What works:

Lead with a summary table by property, not by window, showing how many windows and rooms meet the guidance and how many do not, with the magnitude of the shortfalls banded. Then give the full window-by-window results as an appendix.

Include a plan showing each assessed property numbered, with a colour key for the outcome, so the officer and the committee can find their own street. Describe existing conditions honestly, giving the absolute values as well as the percentage changes, because a window dropping from 12 per cent vertical sky component to 9 per cent is a different story from one dropping from 32 to 24.

And write a short section explaining what the numbers mean for the people living there. That paragraph is what the committee reads.

Keep the technical appendices in a form the council's reviewer can interrogate. Supplying the model, the assumed reflectances and the window schedule alongside the results turns a review into a checking exercise rather than a rebuild, and it usually saves several weeks on a scheme of any size.

Where the scheme also needs a townscape and visual impact assessment, coordinate the two, because the viewpoints and the receptor list often overlap and the same three-dimensional model can serve both.

Where scoping disputes arise at committee

Three recur. A property left out with no explanation. A baseline that quietly includes a consented scheme nobody has built, making the existing situation look worse than it is. And amenity space assessed as a single average when half of it is in permanent shade.

All three are avoidable at scoping, and all three are fatal to the credibility of an otherwise careful daylight and sunlight assessment. Where gardens and courtyards are the sensitive receptors, a separate overshadowing study presented alongside the window results usually reads better than burying the shadow work in an appendix.

To see which reports a site is likely to trigger before you commission any of them, enter your postcode on PlanScope.

For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.