Setting daylight targets on a dense urban site

Urban daylight target setting is the part of a dense-site application that most often goes wrong, and it goes wrong early. If your consultant tests a central London scheme against the 27 per cent vertical sky component figure from BRE 209 and reports the failures, you have produced an objection document rather than an assessment. The guidance itself says its numerical values should be interpreted flexibly, because daylight is one of several factors in site layout design. The work is to decide what target is right for this site, justify it, and get the authority to accept it before the application is submitted.

That decision is a planning judgement supported by technical evidence. It is not something a modeller can settle alone.

Why a flat numerical target fails in dense contexts

The BRE figures were derived for suburban conditions with obstruction angles of around 25 degrees. A 27 per cent vertical sky component corresponds roughly to that geometry. In a street of five storey mansion blocks with a 15 metre carriageway, the obstruction angle from a ground floor window is already far higher, and the existing vertical sky component may be in single figures before anything is built.

Apply the standard target in that setting and two things happen. Windows that are already below target are recorded as failing, which tells the reader nothing about the effect of the proposal. And the relative test, where a value below 27 per cent and below 0.8 times its former value is treated as noticeable, becomes the only meaningful measure.

The same problem affects sunlight. The annual probable sunlight hours criteria, 25 per cent of annual hours with at least 5 per cent in the winter months, were set for a context where a south-facing window has a reasonable view of the sky. In a dense block the test outcome is often decided by the orientation of the street rather than by the massing you are proposing.

The flexibility BRE 209 gives you

The 2022 edition of BRE 209 is explicit that its guidelines are advisory and that alternative target values may be appropriate in dense urban areas, provided the alternative is justified. That sentence is the whole basis for context-based target setting, and it is worth quoting in the assessment rather than paraphrasing.

Flexibility is not the same as abandoning the numbers. An alternative target still has to be a number, derived from something, and tested consistently across the scheme. Three derivations tend to hold up:

• The prevailing condition in the surrounding townscape, measured from a sample of comparable existing windows in the same street pattern.
• The condition implied by the development plan, where an allocation or a tall building zone sets a density or height that cannot be delivered at suburban obstruction angles.
• The condition in a consented or recently completed scheme of similar form in the same borough.

Each needs the sample defined and the method stated. A target derived from six windows in one building is not evidence of a prevailing condition.

Building an evidence based alternative target

Start with the survey. For an existing-window assessment you need accurate room layouts for the neighbouring properties, and in dense contexts that usually means a measured survey or a careful reconstruction from floor plans rather than an assumption of 4 metre deep rooms. Room depth drives the no-sky line result, so an assumed layout can move the outcome more than the massing does.

Then set out the alternative target with its derivation in the same place. A short table listing the metric, the standard value, the proposed site-specific value and the evidence for it is easier for an officer to accept than the same material spread across a report.

The relative tests should almost always be retained. They measure what the proposal does, which is the question in front of the decision maker, and giving them up weakens the argument that the absolute target needed adjusting.

Choosing the right baseline

The second decision is what the proposal is compared against. The default is the site as it stands. Where the site holds a consented scheme that has not been built, or a lawful building that could be rebuilt to a similar envelope, a second baseline is legitimate and often more informative.

A notional compliant massing is the harder case. Modelling a hypothetical scheme that complies with the plan's height and setback policies, then comparing your proposal against it, can be persuasive where the plan positively allocates the site for tall buildings. It is also easy to game, and officers know it. If you use one, publish the massing assumptions and keep them conservative.

Where the existing site is cleared or in low-density use, be straightforward about it. A cleared site produces flattering baseline results, and an assessment that leans on that without acknowledging it loses credibility on the first read. A daylight and sunlight assessment that states its baseline choice plainly tends to survive scrutiny better than one that buries it.

Agreeing the approach with the authority

Target setting belongs in pre-application discussion, and the output should be written. Ask for a technical note exchange rather than a verbal indication at a meeting, because the officer who attends the meeting is not necessarily the one who writes the committee report.

What to put in front of the authority:

1. The site's existing daylight condition, measured rather than described.
2. The proposed alternative targets with their derivation and sample.
3. The baseline or baselines you intend to use.
4. The internal daylight approach for the new dwellings, under BS EN 17037 and its UK national annex.
5. A short list of the neighbouring properties you will assess and why those and not others.

Many authorities appoint their own daylight consultant for larger schemes. That review is useful rather than adversarial if the method has already been agreed, because the argument narrows to inputs and modelling rather than to first principles. Where a neighbour later objects, an agreed method also makes the response straightforward, and the usual grounds for a neighbour daylight objection are easier to answer when the target was settled in advance.

Writing the justification down

The justification section is the part that gets read by the planning officer rather than by the consultant. Keep it to one page and make it answer three questions: why the standard value does not describe this context, what value you have used instead, and what the proposal does to the existing condition.

Avoid the temptation to present the alternative target as a relaxation. It is a different measure of the same thing, chosen because the standard one does not discriminate between a good and a bad scheme in this setting. Say that, with the evidence, and the policy conclusion follows.

An illustrative example

A nine storey residential scheme on a corner site in an inner London borough faces a four storey terrace across a 12 metre street. Existing vertical sky component at the terrace's ground floor windows ranges from 8 to 14 per cent. A sample of 40 comparable windows on similar streets within 500 metres gives a prevailing range of 7 to 16 per cent. The assessment adopts that range as the context target, retains the 0.8 relative test, and reports that 34 of 38 assessed windows stay within it. This example is illustrative and not drawn from a decided case.

What happens if the target was never agreed

An unagreed target is the weakest position in a daylight case. At committee, the officer has to choose between your number and the one in the published guidance, and the published guidance is easier to defend. Objectors will quote the 27 per cent figure, and the committee will hear it as the standard.

On appeal, inspectors have accepted lower absolute values in dense urban settings where the evidence supported it, and those decisions are indicative of how the issue has been approached rather than binding precedent. The pattern in the reasoning is consistent: the inspector wants to see the context measured, not asserted. Internal daylight for the proposed dwellings gets the same treatment, and an internal daylight assessment of the proposed units carries more weight when the same method discipline is applied to both sides of the scheme.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and note that daylight modelling for a submission should be prepared or reviewed by a suitably qualified assessor.

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