Daylight in a dense urban block: realistic targets

Urban daylight targets taken straight from the BRE guidance will fail a dense city site, and they were never meant to pass it. The numerical values in Site Layout Planning for Daylight and Sunlight are advisory, and the guide itself says they should be applied flexibly according to context. The work in a dense block is not hitting 27 per cent vertical sky component. It is showing the decision maker what a reasonable outcome looks like in that particular context and why.

That argument is won or lost on the baseline. Assess a six storey proposal against a two storey neighbour on a site allocated for twelve storeys and every number looks catastrophic. Assess it against what the plan expects on that land, and the picture changes without a single figure being fudged.

Why BRE figures assume suburban conditions

The familiar numbers come from a low rise context. A vertical sky component of 27 per cent at the centre of a window is roughly what an unobstructed window in a two storey suburban street receives, and the 0.8 retained value rule was written for incremental change in settled residential areas.

Apply those directly in a city centre and two problems appear. Existing windows often start well below 27 per cent because they already face a tall building across a narrow street, so the absolute test is failed before you draw anything. And the 0.8 ratio bites hard where the starting value is low, since a drop from 12 per cent to 9 per cent breaches the ratio while representing a small absolute change that nobody in the room would notice.

The 2022 edition of the guidance is clearer about this than earlier editions, and it brings in the internal illuminance approach from BS EN 17037 for new accommodation. The UK national annex sets minimum target illuminances by room type, with bedrooms, living rooms and kitchens each treated differently, over a proportion of the space for a proportion of daylight hours. Use those for the proposed dwellings and keep the sky component tests for effects on neighbours.

Using an alternative baseline

An alternative baseline replaces the existing physical situation with a defensible notional one. Three are used in practice.

The existing condition, which is always reported because it is the factual starting point. A notional compliant envelope, being the massing that the development plan and adopted design guidance would support on the site. And a mirror massing, where the surrounding built form is reflected onto the application site to test whether the neighbour's current daylight depends on an anomaly.

None of these replaces the existing baseline. They sit alongside it, and the assessment reports both. The argument is that the existing condition overstates the neighbour's entitlement because it depends on a site being underdeveloped, and that the notional case shows the daylight the neighbour could reasonably expect. Our note on the mirror image baseline goes through the mechanics.

Be careful with the compliant envelope. If it is drawn generously it reads as advocacy. Build it from the adopted plan, the site allocation, any adopted design code and the prevailing heights, and show the working.

Mirror massing and compliant envelope tests

Mirror massing is most persuasive where a neighbour has unusually large windows, an unusually shallow room, or an outlook over a gap that exists only because a site has been vacant for thirty years. Reflecting the neighbouring block onto the application site tests whether the current values are a product of the urban grain or a product of the gap.

The compliant envelope test works differently. It asks what the plan expects and models the daylight effects of that, then compares the proposal against it. Where the proposal performs better than the plan compliant scheme, that is a strong point and it should be stated in one sentence rather than buried in an appendix.

Both tests invite the same response from an objector: the notional building does not exist. Answer it directly. The tests inform the weight to attach to the numbers rather than replacing them, and the decision maker is entitled to consider what would happen anyway.

Reporting against context rather than in isolation

Context reporting means giving the reader something to compare against. A vertical sky component of 14 per cent means nothing on its own. The same figure, presented alongside the typical range for windows on that street and on comparable streets in the borough, means a great deal.

Three techniques help.

1. Report the absolute value, the retained ratio and the actual change in percentage points, so a small absolute change is not hidden behind a failed ratio.
2. Give the room use for every affected window, because a bedroom and a stairwell carry different weight, a point developed in our note on room use assumptions.
3. Present a contextual survey of existing values in the immediate area, so the decision maker can see what normal looks like there.

Then write the conclusion in plain words. A table of pass and fail marks tells a decision maker nothing. A paragraph saying which rooms are materially affected, by how much, and what that means for the people using them tells them everything.

Policy support for flexibility

The guidance itself provides the primary support, since it states that its numerical values are advisory and should be interpreted with reference to context. National design policy encourages well designed places at appropriate densities rather than compliance with a single metric, and the National Design Guide takes the same approach.

In London, the London Plan expects housing quality and standards to be met while supporting a design led approach to optimising site capacity, and the Mayor's guidance addresses how daylight and sunlight should be assessed in dense contexts. Outside London, some authorities have adopted local guidance recognising the same point, and some have not, so check the development plan before assuming flexibility is available.

Where flexibility is refused

Flexibility is not a licence. Inspectors and officers push back in recognisable circumstances.

Where the affected rooms are living rooms and bedrooms in existing homes and the reduction is large in absolute terms. Where the assessment relies on a notional baseline that is plainly more generous than anything the plan supports. Where the room use assumptions have been chosen to suit the applicant. Where an alternative layout would achieve a similar quantum with materially less harm, and the applicant has not tested it. And where the proposed dwellings themselves are poorly lit, because harm to future occupiers attracts less sympathy than harm to neighbours.

Test one or two alternative massing options and report them. An assessment that shows the scheme has been optimised for daylight, rather than merely assessed, is much harder to argue against.

Structuring the assessment

Set the report out in the order a decision maker reads it: methodology and the guidance relied on, the baseline scenarios with the reason for each, effects on neighbouring properties window by window and room by room, daylight and sunlight within the proposed development, overshadowing to amenity areas, and then a conclusion in plain language.

Where a neighbouring property cannot be entered, say so and explain the assumptions used, following the approach in our note on assessing daylight to a property you cannot access. Where amenity space is affected, the sun on ground analysis belongs in an Overshadowing Study rather than being folded into the window results.

A Daylight and Sunlight Assessment of this kind is specialist work, and councils expect a suitably qualified author. Professional review of any draft is strongly advised before submission.

Illustrative example. A seven storey scheme faces a four storey block across a 16 metre street. Existing vertical sky components on the facing elevation range from 11 to 18 per cent. The proposal reduces them to between 8 and 14 per cent, failing the 0.8 ratio at eleven windows. A mirror massing test shows values between 9 and 13 per cent, so the proposal performs comparably to the reflected context. The officer accepts the effect as acceptable in that location. The figures are illustrative.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.