Cumulative daylight assessment with consented schemes

A cumulative daylight assessment with consented schemes answers a question the BRE guidance does not ask directly: what happens to these windows if the neighbouring permission is built as well as yours. Boroughs with several towers in the pipeline now request it routinely, usually as a second scenario alongside the conventional existing-baseline results. The trap is treating it as the primary test. It is additional information, and how you present it decides how much weight it carries against you.

Why a second scenario gets requested

The standard approach measures the change from the existing built form to the proposed one. Vertical sky component, the no-sky line and annual probable sunlight hours are all compared with what the neighbour has today.

In an area with three consented but unbuilt schemes, that comparison can mislead in both directions. It overstates the harm your scheme causes, because part of the loss will happen anyway when the neighbouring consent is implemented. It can also understate the eventual condition of the affected rooms, because the combined effect of four schemes is worse than any one of them.

Officers and objectors both notice this. A consultee asking for a cumulative run is usually trying to understand whether a room ends up unusable, rather than trying to catch you out.

The BRE guidance itself is advisory, and it says so. Its numerical targets were written for suburban conditions and it invites authorities to apply them flexibly where the existing townscape is dense. That flexibility is what makes the cumulative scenario useful, because it moves the discussion from a failed ratio to the actual condition of a room.

Finding the schemes that belong in the scenario

Build the list from the planning register rather than from memory. Search the authority's register for the surrounding area, then filter for permissions that are extant, unimplemented and capable of affecting the same receptors.

For each candidate, record the reference, the decision date, the expiry date, whether a material start has been made, whether reserved matters remain outstanding, and whether the section 106 agreement has been completed. In London, add a check of the Mayor's planning datahub for referable schemes.

Three categories then fall out.

• Consented, section 106 signed, no expiry risk, demolition under way: include it, and expect the authority to treat it as near certain.
• Consented and unexpired but with no start and reserved matters outstanding: include it as a sensitivity, and say so.
• Resolution to grant with no decision issued, or an application still undetermined: normally exclude, but disclose that you considered it.

Permission time limits matter here. A full permission carries a three-year limit unless the authority directed otherwise, and a scheme approaching that date without a material start is a weaker component of the baseline than one with a demolition contract already under way.

Choosing the cut-off date

Fix a date, state it, and apply it consistently across every scenario. The date the modelling was run is the usual choice, and it should sit close to the submission date rather than six months before.

Then set out the search you did: which register, which radius, which date range, which filters. A cumulative run whose scheme list cannot be reproduced is worth very little, because the first question at any hearing is why a particular consent was left out.

The radius is a judgement. Anything capable of obstructing the sky vault seen from the assessed windows should be in, which in dense urban areas usually means a wider search than the immediate block.

Agree the list with the authority if you can. A short email at pre-application stage confirming which consents the officer expects to see modelled costs nothing and removes the most common reason a cumulative run is rejected, which is that it left out the scheme the objectors care about most.

Running with and without, and reporting both

Model three states rather than two.

1. Existing baseline, existing surroundings as built.
2. Cumulative baseline, existing surroundings plus the included consents.
3. Cumulative baseline plus the proposed development.

The comparison the authority wants is 2 against 3, because that isolates your scheme's contribution in a world where the neighbours are built. The comparison policy requires is 1 against 3, and against 1 with your scheme alone. Report all of them in the same table for the same set of rooms, using the same room-use assumptions.

Do not present only the scenario that flatters the scheme. Reviewers read the appendices, and a report that omits the existing-baseline comparison reads as an attempt to hide it. The Daylight and Sunlight Assessment should carry every scenario with the assumptions stated, and the sunlight results should be reported alongside where gardens and amenity space are affected, which is where an Overshadowing Study does its work.

Keep the room-use assumptions stable across scenarios. Changing a room from a bedroom to a kitchen between runs, even with a good reason, invites the accusation that the results were engineered.

How much weight a consented scheme deserves

Decision makers weigh the likelihood of implementation. A permission with a signed agreement, discharged conditions and a contractor on site is close to certain. A permission granted four years ago on a site with no visible activity is not.

The honest position is that this is a matter of planning judgement, and the assessment's job is to inform it rather than decide it. Set out the implementation evidence for each included scheme and let the officer weigh it. Where a single consent drives most of the cumulative loss, run a sensitivity without that scheme so the report answers the obvious question before it is asked.

Appeal decisions on this point are indicative rather than binding. Inspectors have been willing to take a realistic view of what will actually be built, and they have also been unimpressed by applicants who treat an unimplemented consent as if it were a building.

When the baseline moves mid-determination

It happens. A neighbouring scheme is approved in week nine, and your carefully dated scenario is out of date.

Deal with it in a short addendum rather than a reissued report. State the new consent, its reference and date, whether it materially changes the results, and provide a revised table only for the affected receptors. Officers prefer a two-page note they can append to the committee report over a 90-page replacement three days before the meeting.

Where the new consent changes the conclusion, say so plainly and early. A late correction that comes from the applicant is survivable. The same correction raised by an objector at committee usually is not.

If the scheme's own internal daylight is also in question, the method for assessing daylight inside your own proposed flats uses different metrics again, and the position on average daylight factor and where it still applies is worth checking before you agree a methodology statement with the authority.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Draft the supporting statements for a scheme with neighbouring consents on PlanScope, then have the modelling run and reviewed by your daylight consultant.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.