Mirror massing and the notional compliant envelope

On a dense urban site the absolute daylight targets in the Building Research Establishment guidance are often unachievable by any developable scheme, and a bare failure against them does not resolve the planning question. The response practitioners use is a comparison: model a notional envelope that the site could reasonably have carried, test its effect on the neighbouring windows, and compare that with the effect of the scheme actually proposed. Where the proposal performs no worse than the notional envelope, the argument is that the harm arises from the urban context rather than from this design.

The method has real force and it is also the most abused technique in daylight evidence. A notional envelope drawn generously can justify almost anything, and inspectors and objectors' consultants know it.

Why a bare failure is not the end of the case

The guidance itself acknowledges that its numerical values are advisory and that in dense urban areas, where buildings sit close together and existing sky visibility is already low, applying the targets rigidly may be inappropriate. It contemplates setting alternative target values based on the existing and emerging context.

That is the doorway. It is not a general licence to ignore the numbers, and the weight the guidance carries locally depends entirely on how the development plan has adopted it, which varies a great deal, as our note on how local plan amenity policies use BRE 209 sets out.

Two preconditions have to be satisfied before the comparison is worth running. The site must be in a context where the absolute values cannot in fact be met by a policy-compliant scheme, which means you need to demonstrate that rather than assert it. And the receiving windows must have a sky visibility that is already constrained by the existing urban form, not by the proposal alone.

Where a site is open and the neighbours currently enjoy good daylight, the comparison approach is the wrong tool and using it reads as an attempt to avoid the result.

Building the notional compliant envelope

There are two families of notional envelope, and the distinction matters.

The first mirrors the existing context. A mirror massing envelope reflects the massing of the building opposite, or of the adjoining buildings on the same street frontage, on the basis that a symmetrical urban form is what the street would produce. It is straightforward to derive and easy to explain, and it works best on a consistent street where one plot is anomalously low.

The second is derived from policy. A notional policy-compliant envelope is built from the parameters the development plan sets for the site: any allocated height or storey range, the design code or tall buildings policy, prevailing building lines, required setbacks at upper levels, and daylight and sunlight standards for the proposed accommodation. This is the stronger basis where a site allocation or design code exists, because the envelope is not your invention.

Derive the envelope transparently. State every parameter used and its source, with the policy reference. Show the envelope as a three-dimensional massing drawing alongside the proposal, at the same scale and from the same viewpoints. And constrain it by the things that would actually constrain a real scheme: daylight to the proposed units within the envelope itself, separation distances, access, and any heritage setting policy.

An envelope that fills the plot to the policy maximum in every dimension simultaneously is not a scheme anybody could build, and an inspector will say so.

Choosing the comparison basis

Pick the basis before you run the model, record the choice, and do not change it when the results come back.

Three honest bases. Compare the proposal against the notional envelope on the same metrics, window by window. Compare both against the existing condition, so the reader can see the absolute starting point. Or compare the proposal against the existing condition and report the notional envelope result as context rather than as the test.

The third is the most defensible and the least used. It keeps the existing condition as the baseline, which is what the guidance actually measures from, and uses the notional envelope to show what a reasonable alternative would have done. That framing is harder to attack because it does not substitute a hypothetical scheme for the real baseline.

Where sunlight to gardens and amenity space is also at issue, run the same comparison on sun hours on ground through an overshadowing study, since the result there often differs from the window-based findings and a scheme that performs well on one can perform badly on the other.

Where the method is accepted and where it is not

Acceptance turns on three things: whether the context justifies departing from absolute values, whether the envelope is derived from something external to the applicant, and whether the absolute results are reported honestly alongside the comparison.

It is generally accepted in central and inner London, in major regional city centres, and on sites with an adopted allocation or design code that sets height parameters. The London Plan's approach to daylight within development and to neighbouring amenity supports a contextual application of the guidance rather than a mechanical one, and boroughs apply it that way.

It is generally not accepted on suburban sites, on sites where the neighbouring property is a two-storey house with good existing daylight, where the notional envelope exceeds anything the plan supports, or where the assessment reports only the comparison and buries the absolute figures in an appendix. A refusal reason that says the applicant has substituted a hypothetical scheme for the policy test is the usual outcome in those cases.

My own view is that the method is most persuasive when it is used to explain a result rather than to replace one. A report that says the proposal reduces the sky component at a particular window to 14 per cent, that the existing value is 19 per cent, and that a policy-compliant envelope would produce 15 per cent, has told the decision maker something useful. A report that says only that the proposal performs better than a notional envelope has told them nothing they can test.

Presenting the comparison honestly

Structure the reporting so the absolute position comes first.

Report the existing values, then the proposed values, then the change and the ratio against the guidance criteria, window by window and room by room. Only then introduce the notional envelope, with its derivation, its parameters and their sources. Then give the envelope's results on the same metrics and the same windows. Finally give the three-way comparison in a single table.

Include the windows where the proposal performs worse than the notional envelope. There will usually be some, because the two massings differ in shape as well as volume, and a report with no such cases looks curated. Explaining why a particular window is worse under the proposal and why that matters less than the overall position is a stronger argument than pretending it does not happen.

State the model inputs, the obstruction survey source, the assumed room uses and how they were established. A full daylight and sunlight assessment carries all of this, and the comparison section should read as one part of it rather than as the conclusion.

Objector responses to the approach

Anticipate four objections, because they arrive every time.

That the notional envelope is undeliverable, which is answered by constraining it with internal daylight, access and separation requirements and showing that work. That the envelope has no planning status, which is answered by deriving it from adopted policy rather than from judgement. That the absolute harm remains whatever the comparison shows, which is a fair point and should be met by quantifying the absolute harm rather than deflecting it. And that the approach inverts the burden of proof, treating a hypothetical scheme as the baseline.

The last is the serious one. The answer is to keep the existing condition as the stated baseline throughout, so the comparison is additional information rather than a substitute test. Objectors' consultants raise these points competently, and the evidence that carries weight on the other side is described in our note on objecting on daylight grounds.

Using it in a statement of case

In an appeal statement, put the comparison in the planning balance rather than in the technical conclusion.

A workable sequence: the policy framework and how the plan adopts the guidance, the existing context and why absolute values are unachievable, the absolute results with the harm quantified, the notional envelope and its derivation, the comparative results, the scale of residual harm, and the benefits weighed against it. That order keeps the technical findings separate from the judgement, which is where an inspector wants them.

Where the authority's own consultant has modelled the scheme, agree a joint statement of agreed facts on the inputs if the inspector allows it. Most daylight disputes at inquiry turn out to be disputes about room uses and obstruction geometry rather than about the method, and settling those narrows the case considerably.

Daylight modelling is specialist work and councils expect a suitably qualified author, with professional review strongly advised before submission. Enter your postcode on PlanScope to draft the planning statement that carries the balance.

This is general guidance, not legal advice. BRE 209 is advisory rather than statutory, so check your authority's adopted amenity policy and take advice from 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. For specialist technical reports, qualified professional input may be required.