Mirror massing and alternative scheme comparisons

A mirror massing daylight comparison is an argument, not a test. You build a notional envelope on the application site that reflects what the development plan envisages, run the same daylight analysis against it, and compare the results with your proposal. If the neighbours' windows perform much the same under both, the argument is that the shortfall reflects the site's context and the plan's own aspirations rather than anything particular to your design.

No published guidance prescribes the technique. The BRE document recognises that its numerical criteria may need to be applied flexibly in dense urban settings, and the comparison grew out of that recognition. So its weight depends entirely on how defensible the notional envelope is.

Why the comparison gets produced

Three situations generate it.

The first is the atypically sensitive neighbour. A single-storey building with large windows facing an open site will show large percentage losses against almost any development, because its existing vertical sky component is unusually high. The numbers look alarming and the underlying position is that the neighbour has been enjoying the daylight of a vacant plot.

The second is the cleared or underdeveloped site. Where the existing baseline is a car park or a demolished building, every proposal produces a large change, and the comparison puts that change into the context of what the plan intends for the site.

The third is the tall building in a designated cluster, where the adopted policy positively directs height to the location and the surrounding buildings were consented on that basis.

In each case the comparison answers the question an officer is actually wrestling with: is this scheme worse than what the plan asked for, or is the plan's aspiration itself the source of the impact. That is a legitimate question and the analysis can help answer it. The related question of comparing against a mirrored version of the proposal on a facing site is a different technique, covered in mirror image baseline and existing obstructions.

Building the notional envelope

Every parameter has to come from a source you can cite, and the sources have a hierarchy.

Where the plan is silent on a parameter, say so and explain the assumption you made. A silence handled openly is far less damaging than a number inserted without comment.

Two disciplines make the difference. Build the envelope before you know the answer, so that the parameters are not reverse-engineered from the result you want. And build it as a plausible building rather than a solid block: a notional scheme with no set-backs, no articulation, no cores and a flat parapet at maximum height will produce worse results than any real scheme, which is precisely why objectors attack it.

Document the process as you go, with the date each parameter was fixed. That record is what allows you to say the envelope was not shaped by the outcome.

Comparing proposed against notional results

Run the two models identically. Same weather data or sky model, same room uses, same internal reflectances, same treatment of trees and existing obstructions, same window schedules, same surveyor's base model.

Report three sets of numbers rather than two: the existing baseline, the notional scheme, and the proposal. Then report the proposal against the baseline, which is the conventional assessment, and the proposal against the notional, which is the comparison.

Present them at room level as well as in the summary tables. An aggregate statement that the proposal performs comparably to the notional scheme can conceal a pattern where the proposal is better at the top of a neighbouring building and materially worse at the lowest two floors, and the lowest two floors are where the affected residents live.

Where the proposal performs better than the notional scheme on some windows and worse on others, say so in those terms. A comparison reported as uniformly favourable when it is mixed is the fastest way to lose the argument, since the other side's consultant will run the same model.

Where the comparison helps and where it misleads

It helps where the notional scheme is one the authority would recognise as consistent with its own plan, and where the proposal's results are similar or better. In that configuration the analysis does real work in the planning judgement.

It misleads in four recurring ways.

Where the envelope is a straw man, built to fail, so that any proposal looks reasonable beside it. Where the notional scheme is treated as a permission baseline, as though the applicant were entitled to build it, which is not the law. Where the comparison is used to excuse absolute results that are poor on any view, because a room receiving very little daylight is a poor room regardless of what a notional alternative would have done to it. And where the plan being relied on is out of date or the allocation is being brought forward at a capacity the authority disputes.

The fourth of these is the one worth checking first. If the authority does not accept the capacity or height assumption, the whole comparison collapses, and finding that out at committee is expensive.

Objector criticism of the chosen envelope

Anticipate the attacks, because they are predictable.

The height parameter will be challenged if it comes from an emerging plan, a draft design code, or a reading of policy the authority has not endorsed. The footprint will be challenged if the notional scheme covers ground your own proposal leaves open, since that inflates the notional impact. The absence of set-backs will be challenged. The choice of storey heights will be challenged where a generous floor-to-floor produces a taller envelope than the policy height contemplates.

Two further criticisms are harder to answer. That the comparison is inherently self-serving, because the applicant chose the comparator. And that a notional envelope consistent with policy is not the same as a scheme the authority would have granted, because policy is applied in the round rather than as a set of maximum parameters.

The best response to both is procedural. Agree the parameters with the authority before you model them, in writing, at pre-application. A comparison built on agreed parameters is a different document from one produced unilaterally in a rebuttal, and officers treat it differently.

Recording the assumptions behind the massing

Put a parameters table in the report, not in an appendix nobody reads, with a column for the policy source of each figure and a column for any assumption made where policy is silent.

Include the geometry. Plans, sections and axonometric views of the notional envelope alongside the proposal, at the same scale and from the same viewpoints, so that a reader can see what was compared. A comparison presented only as numbers asks the reader to trust a model they cannot inspect.

State who fixed the parameters and when, and whether they were discussed with the authority. State also what you did not model: below-ground levels, plant enclosures, balustrades, and any part of the neighbouring context that was simplified.

One line worth including: a statement that the notional scheme is an analytical tool and does not represent an alternative proposal or a fallback position. Applicants sometimes omit that, and the omission invites the argument that they are claiming an entitlement.

What weight the comparison carries

In practice the weight varies with three things: whether the parameters were agreed, whether the notional scheme is plausible as a building, and whether the proposal's absolute results are defensible on their own.

Where all three hold, the comparison can carry real weight in the officer's balancing exercise, particularly on sites where the existing baseline is atypical. Where the absolute results are poor, the comparison shifts to being an explanation rather than a justification, and the decision turns on the wider public benefits instead.

The honest limitation is that the technique is contested. It is not in the guidance as a prescribed method, its outputs depend on choices the applicant makes, and different inspectors have given it different amounts of weight on similar facts. Treat it as supporting analysis rather than as the answer, and make sure the primary Daylight and Sunlight Assessment stands up without it.

For the earlier stage, testing massing before the scheme is fixed, see obstruction angles and the early massing test. For the final stage, presenting daylight results a committee can read covers how to lay the comparison out for a lay audience. Enter your postcode on PlanScope to see which reports your site is likely to need.

This is general guidance, not 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.