Mirror massing as the alternative comparator
A cleared site produces huge percentage losses against an existing baseline. A notional comparator can put that in context, if the assumptions behind it survive scrutiny.
Mirror massing daylight comparison exists because of a specific unfairness in the arithmetic. Where an application site is cleared, or carries a single storey shed, the existing baseline gives neighbouring windows an almost open view of the sky. Any building on that site produces a large percentage reduction, even one that leaves the neighbour with perfectly reasonable daylight in absolute terms.
The comparator is an attempt to put that percentage in context. It answers the question a percentage cannot: compared with what could reasonably be expected on this site, how does this scheme perform?
What the comparator is trying to show
Three separate arguments hide under the same technique, and it helps to keep them apart.
The first is a contextual argument. In a dense urban area the daylight a window receives is a function of the street pattern and the prevailing building form, and a site that has been vacant for thirty years does not create a permanent entitlement to that borrowed sky.
The second is a policy argument. Where the development plan, a design code or an allocation sets a height or a density for the site, a scheme that sits within those parameters is doing what the plan asked for, and the daylight consequences of building to the plan are a matter the plan has already accepted in principle.
The third is a fairness argument, which is where mirror massing specifically comes in. If the neighbouring building was itself built up to the boundary, with habitable rooms looking out over the application site, then reflecting that same form back across the boundary shows what the neighbour's own typology implies for everyone around it.
Building the notional envelope from policy and context
An envelope has to be derived from something written down. In rough order of strength:
- An extant planning permission on the site, which has already been judged acceptable by the authority.
- A site allocation or design code with stated heights, storeys or plot ratios.
- An adopted local plan policy on building heights for the area.
- The mirrored massing of the neighbouring building under assessment, taken from measured survey or planning drawings.
- The prevailing parapet height and building line of the block, evidenced from a survey and a townscape analysis.
- A previous building on the site, where records show its form and it was demolished recently.
Anything beyond that list is invention, and reviewers treat it as such. An envelope derived from "what could theoretically be built" with no source is worse than no comparator at all, because it invites the conclusion that the applicant knew the standard results were poor.
Be careful to apply the same constraints to the envelope that the real scheme has to meet. If the site is in a conservation area, the notional envelope cannot ignore that. If a daylight standard applies to the envelope's own occupants, model it. An envelope that would be refused on heritage or amenity grounds is not a fair comparator.
Where the technique is persuasive
It works best in a narrow set of circumstances, and being honest about that helps the argument.
A cleared or under-developed site in a dense urban block, where neighbouring windows currently enjoy sky that no plan ever intended to protect. A neighbour built hard to the shared boundary, sometimes recently, whose own form is the reason its windows are vulnerable. A site with a recent permission for a scheme of similar scale. A site allocated in the plan for a quantum of development that cannot be delivered within the standard numerical guidance.
In those cases the comparator changes the conversation from a percentage to a judgement, which is what the planning balance actually requires. It sits alongside the retained daylight tests rather than replacing them, and the 0.8 rule for retained daylight still frames how the results are reported.
Where it is dismissed as self-serving
The failure modes are predictable:
- An envelope taller or deeper than anything in the area, with no policy source.
- An envelope that conveniently matches the proposed scheme's own massing, which proves nothing.
- A comparator run only for the tests the scheme fails.
- No existing baseline reported, or one buried in an appendix.
- An envelope that ignores heritage, highways, its own internal daylight or party wall constraints.
- A comparison presented as compliance, as though passing against a notional scheme were a policy test.
That last point is the most common drafting error. There is no test that says a scheme is acceptable if it performs no worse than a notional alternative. The comparator is evidence that goes into the balance, and a report that describes it as a pass is overstating its own case.
Inspectors have taken varying views of alternative comparators depending on how well the envelope was evidenced, so treat published decisions as an indication of how the technique lands rather than as authority for using it.
Documenting the assumptions behind the envelope
Write the assumptions as a list in the report, not as prose buried in a methodology section. A reviewer should be able to rebuild the envelope from the list.
- The source for the envelope, cited precisely: permission reference, policy number, code page or survey drawing.
- Height in metres and storeys, with the datum stated.
- The footprint, set-backs and any upper floor recessions, with a plan.
- Whether the envelope was tested for its own internal daylight, and the result.
- Surface reflectances and any glazing assumptions, matching those used for the real scheme.
- What has been deliberately excluded, such as balconies or plant enclosures, and why.
- A statement that the envelope is notional and is not put forward as a consentable scheme.
Illustrative example. A four storey Victorian terrace built to the boundary of a cleared plot, with rear habitable windows facing the plot. The existing baseline produces losses above the guidance for eight windows. The mirrored massing comparator, built from a measured survey of the terrace itself, shows that the same losses would arise from a building matching the neighbour's own form, and that the proposed scheme performs slightly better than that mirror because its upper floor is set back 2 metres. Reported with the existing baseline first, that is a fair and useful piece of analysis. Reported alone, it looks like an attempt to bury the standard results.
Combining it with the existing baseline
Report in this order. Existing condition first, in full, room by room, with the guidance values and every result that falls short. Then the comparator, with its derivation. Then a short section that explains what the comparison shows and what it does not.
A table with three columns, existing, proposed and notional, for each affected room, does more work than several pages of narrative. It also makes clear where the comparator makes no difference, which happens more often than applicants expect and is worth saying out loud.
The same logic applies to overshadowing of gardens and amenity space, where an Overshadowing Study can be run against the notional envelope, and to internal results, since internal daylight against neighbouring properties is part of the same weighing exercise.
Presenting the comparison honestly
Four habits separate a comparator that carries weight from one that does not.
Run it for every test reported, including the ones where it does not help. State clearly that the standard guidance results are the primary analysis. Quantify the difference the comparator makes rather than describing it as showing the scheme is acceptable. And name the honest limitation, which is that a notional envelope is a construct, and its weight is a matter for the decision maker.
Officers are not naive about this. A report that volunteers the weakness of its own comparator is read more generously than one that does not, and what a case officer checks first is usually whether the existing baseline is there and complete.
Where the comparator is doing heavy lifting on a contentious scheme, expect the authority to appoint its own reviewer, and expect the envelope to be rebuilt independently. Design it to survive that rather than to produce the best number.
PlanScope drafts the narrative, methodology and policy sections of a Daylight and Sunlight Assessment from your scheme information, with flags where modelling results have to be inserted. 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, and the modelling behind any comparator should be run by a suitably qualified assessor.
Draft your assessment narrative on PlanScope, then send it for professional review with the modelling attached.
Frequently asked questions
What is mirror massing in a daylight assessment?
A notional development on the application site that reflects the massing of the neighbouring building being assessed, mirrored across the shared boundary. It tests whether the neighbour's daylight expectations are consistent with its own built form. It is most telling where the neighbour was built hard to the boundary with windows that depend on sky over the application site.
Is an alternative comparator accepted by planning authorities?
Sometimes, as context rather than as a substitute for the standard tests. The BRE guidance recognises that numerical targets need to be applied with judgement in dense urban settings, and comparing against a notional scheme is one recognised way of doing that. Authorities accept it where the envelope is credible and reject it where it looks constructed to produce a result.
Can I use an extant planning permission as the baseline?
You can present it as a comparator, and it is often the strongest one available because somebody has already judged it acceptable. Report the existing condition as the primary baseline and the consented scheme alongside it, stating whether the permission is still capable of implementation and when it expires.
Why do cleared sites produce such large percentage losses?
Because the existing baseline is an open site with almost unobstructed sky, so any building produces a large proportional reduction even where the result remains reasonable in absolute terms. This is the situation where absolute values, the character of the area and a comparator all deserve weight alongside the percentage change.
Does a comparator help with sunlight as well as daylight?
The same logic applies to annual probable sunlight hours and to overshadowing of amenity space, and the comparator should be run for every test being reported rather than only the one that produces the most helpful answer. Selective use is the fastest way to have the whole exercise discounted.
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.