Mirror image and policy compliant schemes as a comparator
Where BRE 209 results fail for contextual reasons, a mirror image massing or a policy compliant alternative supplies the missing baseline. Here is how to build one that an inspector will accept.
A comparator scheme is a way of answering the question a raw set of daylight results cannot: compared with what? Where an existing building sits close to a boundary, or a neighbour's windows face a flank wall three metres away, absolute percentages against the BRE 209 targets will show failures that no development on the site could avoid. A mirror image daylight comparator and a policy compliant alternative are the two techniques used to supply that missing baseline. Both are legitimate. Both are easy to abuse, and inspectors know it.
Why a comparator is used at all
BRE 209 sets numerical guidelines for vertical sky component, no sky line, annual probable sunlight hours and the related tests. The 2022 edition is explicit that these are guidelines rather than standards, that they can be applied flexibly, and that context matters in dense urban areas where the existing townscape already departs from them.
That flexibility creates an evidential gap. If a neighbouring window retains a vertical sky component of 19 per cent after development, against a guideline of 27 per cent, the numbers say there is a loss. They do not say whether that loss is a consequence of your proposal or of a building line, a projecting wing or a balcony that the neighbour built themselves.
A comparator fills the gap by modelling a different massing on your site and reporting the same tests against it. The comparison then tells the decision maker something the absolute figures cannot: whether the harm arises from developing the site at all, or from developing it in the particular way proposed.
Constructing a mirror image massing
The mirror image technique takes the existing form on the adjoining site and reflects it across the shared boundary onto the application site. The logic is reciprocity. If the neighbour's own building, mirrored, would produce the same reduction in daylight to its own windows, the window is already designed to a standard that the proposal does not worsen.
It works best in terraced and semi-detached contexts where the urban grain is regular, and on back-to-back relationships where projecting rear wings are the issue. It works badly where the adjoining building is an anomaly, because mirroring an anomaly produces another anomaly.
Two mechanical points decide whether the model is credible. Mirror the built form, including roof pitch, projections and window positions, rather than a simplified box. And mirror it onto the application site in the position the site's own constraints would allow, not floated to wherever makes the arithmetic work.
Where the receiving windows serve single-aspect flats the mirror image rarely helps, because the problem is the neighbour's internal layout rather than the external massing, a situation discussed in daylight to single aspect flats and how to fix them.
A policy compliant alternative as the benchmark
The second technique models a scheme on the application site that complies with every relevant development plan requirement, then reports the daylight effect of that scheme alongside the proposal. If the proposal performs as well as, or better than, a compliant alternative, the daylight effect is not a reason to refuse the proposal in favour of something else.
The word doing the work is compliant. A benchmark massing has to satisfy the whole of the development plan, not the height limit alone. That means plot ratio, separation distances, amenity space, parking, the internal space standard, affordable housing where it affects unit mix, and whatever the local design code requires. A benchmark that hits the height parameter while ignoring the separation distance policy is not policy compliant, and an inspector will say so.
| Technique | What it tests | Where it fails |
|---|---|---|
| Mirror image | Whether the neighbour's own form would cause the same loss | Anomalous adjoining buildings, irregular grain |
| Policy compliant alternative | Whether a plan-compliant scheme would perform better | Benchmark that complies selectively |
| Existing building comparison | Change against the current situation on site | Cleared sites, long-vacant buildings |
| Contextual benchmark | Performance against the prevailing local standard | Areas with no consistent prevailing pattern |
The existing building comparison deserves a note of caution. Where the site holds a building that already restricts the neighbour's light, the change from the existing situation is a fair measure. Where the site was cleared five years ago, comparing against the demolished building invites the objection that the neighbour has enjoyed better light since, and that the baseline should be the open site.
Where a comparator helps and where it distracts
A comparator earns its place where the absolute results fail the guidelines and the reason is contextual. Tight urban sites, back-to-back terraces, infill plots between existing buildings, and sites where the neighbour's windows are unusually positioned.
It distracts in three situations. Where the proposal passes the guidelines, in which case the comparator adds pages and invites scrutiny of an argument you did not need. Where the loss is severe in absolute terms, because no amount of comparison makes a window that loses most of its sky acceptable. And where the affected room is a bedroom or a kitchen and the local policy applies a lower expectation anyway.
The reporting weight should follow. Lead with the absolute results against the guidelines, then offer the comparator as context, rather than burying the absolute figures behind a comparison the reader has to take on trust. How a given authority expects that to be presented depends on its own wording, which local plan amenity policies and how they use BRE 209 sets out.
Avoiding a strawman comparator
The failure mode is obvious once named. A comparator massing designed to perform badly makes the proposal look generous, and a decision maker who spots it discounts the whole assessment including the parts that were sound.
Four checks will keep a comparator defensible.
- List every development plan policy the benchmark satisfies, with the policy reference, and state any it does not and why.
- Use the same modelling assumptions for both schemes: reflectance values, glazing transmittance, room uses, obstruction geometry, and the same treatment of balconies.
- Report every window and room in both runs, not a selected subset.
- Have someone who did not build the comparator review whether it is a scheme anyone would actually submit.
The second of those is where most arguments are lost. A proposal modelled with clean glass and a comparator modelled with dirty glass is not a comparison, and the assumption table is the first thing a competent reviewer turns to.
Reporting the comparison honestly
State the purpose of the comparator in a sentence before the results, so the reader knows what question it answers. Then give the absolute results, the comparator results and the difference, in the same table, for the same receptors.
Where the comparator produces an awkward answer, report it. An assessment that concedes two windows perform worse under the proposal than under a compliant alternative, while explaining why that is acceptable in planning terms, is far more persuasive than one where every number falls the right way.
And keep the drawings in the appendix. A comparator described in words and never drawn is an assertion, and the massing model should be reproduced at the same scale and in the same views as the proposal.
How officers and inspectors weigh it
The statutory test is the development plan and other material considerations. A comparator is evidence going to the weight of the daylight objection, and it does not change the test.
In practice officers use comparators to decide whether a daylight consultation response is pointing at a genuine harm or at an artefact of the existing context. Inspectors use them the same way, and they discount comparators where the assumptions differ between runs or where the benchmark is selectively compliant.
The honest limit is this: a comparator explains a shortfall against BRE 209, and it cannot cure one. Where the loss to a habitable room is severe, the comparison becomes background and the planning balance has to carry the case. Set the technique out properly in the Daylight and Sunlight Assessment, keep the Overshadowing Study consistent with it, and expect the assumptions table to be read before the results.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
What is a mirror image daylight comparator?
It models the adjoining building's own form reflected across the shared boundary onto the application site, then reports the same daylight tests against that massing. The logic is reciprocity: if the neighbour's own building mirrored would cause the same loss, the affected window is already designed to that standard. It works best in regular terraced and semi-detached grain.
Does a comparator scheme override BRE 209 guidelines?
No. The guidelines remain the measure, and the comparator is evidence about the weight to give a shortfall. BRE 209 is explicit that its numerical figures are guidelines applied flexibly, particularly in dense urban areas. A comparator explains a shortfall in context; it cannot cure a severe loss of light to a habitable room.
What makes a benchmark scheme policy compliant?
Compliance with the whole development plan, not just the height parameter. That includes separation distances, amenity space, internal space standards, parking, unit mix and any local design code. A benchmark that satisfies height while ignoring separation distance is selectively compliant, and decision makers discount comparators built that way.
Should I include a comparator if my scheme passes BRE 209?
Usually not. A comparator adds length and invites scrutiny of an argument you did not need to make. Lead with the absolute results. Reserve the comparator for cases where the guidelines are not met and the reason is contextual, such as tight infill plots or back-to-back terraces with existing projecting wings.
What assumptions must match between the two models?
All of them. Surface reflectance values, glazing transmittance, room uses, obstruction geometry and the treatment of balconies must be identical in both runs, and every window and room should be reported for both rather than a selected subset. A competent reviewer reads the assumptions table before the results.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.