Mirror image massing tests in daylight assessments
The mirror image test replaces a scheme with a notional contextual form and re-runs the analysis. It is a context test, not a compliance test, and the construction decides the answer.
A mirror image massing test replaces the proposed building with a notional one that reflects the form of what is already there, then re-runs the daylight analysis at the neighbouring windows. It is a context test. It tells you whether the daylight loss a scheme causes is characteristic of the area or particular to the proposal, and it is used in appeal evidence rather than being a criterion in BRE 209.
Used carefully it is persuasive. Used as a substitute for absolute values it invites the criticism that it assumes its own conclusion.
Why the test is run at all
The numerical targets in BRE 209 were derived from suburban conditions. A vertical sky component of 27 per cent at the centre of a window, and the proportional test that a retained value of at least 0.8 times the former value is unlikely to be noticed, both work well where buildings are low and widely spaced.
In a dense urban street they do not describe reality. Existing windows on a narrow street may already receive a vertical sky component well below 27 per cent from buildings that have stood for a century, and any redevelopment of a cleared site will produce a percentage reduction that reads as a failure against the proportional test. The guidance itself recognises that its numerical values should be applied flexibly, with regard to the context in which the site sits.
The mirror image test is one way of giving that flexibility a form the decision maker can weigh. Instead of arguing that the numbers should not apply, it shows what the numbers would be under a form that everyone accepts as contextual.
Constructing the notional massing
The construction has to be defensible, because the choice of what to mirror determines the answer.
Three approaches appear in evidence. Mirroring the building opposite, so the appeal site is modelled with the same height, depth and set-back as the building across the street. Mirroring the building that formerly stood on the site, where the site has been cleared and the historic form is documented. And mirroring the prevailing form of the terrace or block that the site sits within, where the street has a consistent character.
State which you have used and why, with drawings of the notional form alongside the proposal. Model it to the same level of detail as the proposed scheme, using the same surveyed context, the same room assumptions and the same window positions, so the comparison is like for like. A notional massing modelled crudely against a proposal modelled precisely is not evidence, it is an artefact of two different methods.
And model the notional form as a real building would be, with a pitched roof if the context has pitched roofs and with reasonable set-backs. A rectangular extrusion to the height of the tallest neighbour flatters the proposal and will be taken apart.
Using the result to characterise harm
The output is a comparison, and the honest way to present it is as a comparison of three states: existing, notional contextual, and proposed.
If the proposed scheme produces results close to the notional contextual form, the argument is that the daylight conditions the neighbours would experience are those the street generates anyway, and the harm is not attributable to overdevelopment. If the proposal is materially worse than the notional form, the test has told you something useful about the design and it is better to know it before submission.
The framing matters. The test characterises harm. It does not eliminate it, and a report that presents the comparison as demonstrating compliance is overstating what the method can do. Harm to a neighbour's daylight remains harm even where it is typical of the area; the point is what weight it carries in the balance.
Pairing this with an assessment of the existing baseline is essential, and our note on daylight assessment in low baseline conditions covers the related problem of proportional tests applied to already poor conditions.
How the approach is received
Inspectors have accepted contextual and alternative massing analysis as a material consideration in appropriate cases, particularly in dense urban settings where mechanical application of the numerical targets would prevent any redevelopment. They have also given it little weight where the notional form was contrived, where absolute retained values were very low, or where the appellant relied on it instead of engaging with the actual effect on individual rooms.
Appeal decisions are indicative of how inspectors have approached the issue. They are not binding precedent, and quoting one as though it were will not help. Cite them for the reasoning rather than the result, and be honest about the differences between that case and yours.
The safest assumption is that the test earns weight in proportion to how transparently it is constructed.
The criticisms, stated fairly
Four objections come up, and each has force.
The first is circularity. The test assumes the mirrored form would be acceptable, which is the question at issue. Where the building being mirrored is itself an overdevelopment, the comparison legitimises it.
The second is the ratchet. If each new scheme is tested against the last thing built, standards drift downwards over time and no individual step appears harmful.
The third is that it says nothing about absolute conditions. A room receiving a vertical sky component of 8 per cent is poorly lit whether or not the neighbouring form is contextual, and residents experience the absolute condition rather than the comparison.
The fourth is selection. The assessor chooses what to mirror, and different choices produce different answers on the same site. That is why the reasoning for the choice has to be in the report rather than in the assessor's head.
Presenting it without overstating it
Put the test where it belongs in the report structure: after the primary analysis, clearly labelled as a context test, with its assumptions listed.
- Report the primary BRE 209 analysis first, in full, with retained vertical sky component and no sky line values for every assessed window and room.
- Identify the windows where the proposal does not meet the numerical guidance, and say by how much.
- Introduce the mirror image analysis as additional context, explaining what was mirrored and why.
- Present existing, notional and proposed values side by side in a single table, window by window.
- Draw a conclusion about the character of the effect, not about compliance.
- State the limitations of the method in your own words, before the objector's expert does it for you.
That last step is worth more than it looks. An assessor who sets out the weaknesses of their own method reads as credible on everything else in the document.
Pairing it with absolute values
The strongest daylight cases combine three things: the absolute retained values, the proportional change, and the contextual comparison. None of the three carries a case alone.
Report how many rooms fall below the relevant target and by how much, how many are affected by more than the proportional threshold, what the retained absolute levels actually are, and how those levels compare with the notional contextual form. Then say what the scheme does to mitigate: set-backs at upper levels, light wells, window enlargement on the affected elevation, or reduced depth on the flank facing the sensitive receptors.
Where amenity space rather than windows is in issue, the Overshadowing Study and the sun on ground analysis run alongside, and the two hour test frequently decides garden-based objections in a way the window analysis does not.
A Daylight and Sunlight Assessment is a report type councils expect to be authored by someone suitably qualified, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Is the mirror image test part of BRE 209?
No. BRE 209 does not set it out as a criterion. The technique has developed in appeal evidence, building on the guidance's acknowledgement that its numerical targets were derived from suburban conditions and should be applied flexibly having regard to context. Treat it as supporting analysis rather than as a test the guidance requires.
What should the notional massing be based on?
Usually one of three things: the building opposite, the building that formerly stood on the site where the historic form is documented, or the prevailing form of the terrace or block. State which you chose and why, model it to the same detail as the proposal using the same context and assumptions, and give it a realistic roof form rather than a flat extrusion.
What weight do inspectors give the approach?
It varies with how transparently it is constructed. Contextual massing analysis has been accepted as a material consideration in dense urban settings where mechanical application of the numerical targets would prevent redevelopment. It has attracted little weight where the notional form looked contrived or where the absolute retained values were very low.
What are the main criticisms of the method?
Circularity, because it assumes the mirrored form is acceptable. The ratchet effect, because testing each scheme against the last built allows standards to drift. Silence on absolute conditions, since a poorly lit room is poorly lit whatever the context. And selection, because different choices of what to mirror produce different answers on the same site.
How should the test appear in a daylight report?
After the primary analysis, clearly labelled as a context test. Report the full BRE 209 results first, identify the windows that fall short and by how much, then introduce the comparison with the assumptions listed. Present existing, notional and proposed values side by side, and state the limitations of the method yourself.
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.