The mirror image method when a neighbour's layout is unknown

You cannot measure daylight to a room you have never seen. On most urban schemes the neighbouring internal layouts are unknown, access is refused or ignored, and the assessment still has to be produced. The answer is a stated set of assumptions, applied consistently, recorded in the report and defensible if another consultant re-runs the model.

The mirror image method is the most common of those assumptions, and it is also the most commonly misapplied.

Why internal layouts are often unavailable

Neighbours have no obligation to let a developer's consultant into their home, and many decline once they understand why the request has been made. Some do not answer. Some are tenants who cannot give access without a landlord. Some properties are vacant, or are in the middle of their own building work.

That leaves three sources of information, in descending order of reliability: a measured internal survey, plans obtained from the council's planning history or from Land Registry lease plans, and inference from the exterior.

Always try the second before falling back on the third. Councils publish decision notices and drawings for applications going back decades, and a rear extension application from 2007 will often show the ground floor layout of the property you need. It costs an hour of searching and it removes an assumption you would otherwise have to defend.

What actually needs a layout, and what does not

Half the panic about unknown layouts is misplaced, because the two headline tests do not need one.

Vertical sky component is measured at the centre of the outside face of the window. It is a purely external measure. Annual probable sunlight hours is measured at the same point. Neither requires any knowledge of what sits behind the glass, beyond identifying which windows serve habitable rooms.

Assumptions become necessary for the daylight distribution test, which plots the no sky line across a room and therefore needs the room depth, the room width and the working plane. They are also needed for average daylight factor and for any internal illuminance calculation to BS EN 17037.

So the first thing a report should say is which tests were run and which of them depended on an assumed layout. A report that applies a blanket disclaimer over everything invites the reader to distrust the numbers that were never in doubt.

The mirror image assumption and where it comes from

The mirror image assumption applies where the building being assessed is one half of a symmetrical pair, or one of a run of terraced properties built to the same plan. If you have surveyed number 12 and number 14 is its mirror, you model number 14 as the mirror of number 12.

It is a reasonable assumption for the original plan form of a Victorian or interwar terrace, where the front room, rear room and back addition repeat along the street. It becomes unreliable in three situations.

Where the properties have been extended at different times, which on a suburban street is most of them. Where the run has been converted into flats, because conversion layouts are driven by staircase position rather than by symmetry. And where the original plan itself alternated, as in some byelaw terraces where handing swaps every second pair.

Check the external evidence before relying on it. Window positions, rainwater goods, chimney stacks and the position of the back addition are all visible from the street or from an aerial image, and they tell you whether the plan really is handed.

Assumed room depths and working plane heights

Where no mirror is available, the assessment uses standard assumed dimensions.

• Room depth measured from the window wall, taken as a stated figure and applied consistently across the assessment
• Room width taken from the external window positions and the party wall spacing
• Floor to ceiling height derived from the external survey, storey heights and an assumed floor build-up
• Working plane at 0.85 m above floor level for dwellings, which is the standard height for daylight distribution in a residential room
• Window head and cill heights measured externally rather than assumed

State each figure once, in a table, and reference it from the results section. The reason is practical: a reviewer's first move is to look for the assumption schedule, and a report without one reads as though the assumptions were chosen to suit the answer.

There is a real tension here. A shallow assumed room makes the daylight distribution result look better, because the no sky line has less room to fall short of. A deep assumed room makes it look worse. Whichever way you go, say what you did and why, and run a sensitivity check on the receptors where the assumption changes the outcome.

Recording every assumption in the report

Six items belong in the assumptions section, and they should be readable by a case officer rather than only by another consultant.

1. Which properties were surveyed internally, with dates.
2. Which properties were modelled from council drawings, with the application reference.
3. Which were modelled on the mirror image assumption, and what evidence supports the symmetry.
4. Which were modelled on standard assumed dimensions, with those dimensions stated.
5. Which windows were assumed to serve habitable rooms, and how that was judged from the exterior.
6. What access was requested, from whom, on what dates, and what response was received.

Item five deserves attention. A window with obscure glazing and a small opening light is probably a bathroom, and bathrooms are not assessed under BRE 209. A window at half-landing height is probably a stair. Getting this wrong in the direction of caution inflates the number of affected windows and makes the scheme look worse than it is, which is not the neutral position it appears to be.

When an assumption is too conservative to be useful

There is a point at which caution stops being rigour and becomes noise.

An illustrative example. A consultant models every rear window of a terrace as a habitable room 6 m deep, on the basis that this is the worst case. The result shows widespread failure of the daylight distribution test. On a later site visit to one property, the rear rooms turn out to be 3.6 m deep with a separate rear addition. The failures were an artefact of the assumption.

The honest approach is to model the plausible case, state it, and then show the sensitivity: what happens if rooms are 20 per cent deeper, and what happens if they are 20 per cent shallower. That gives an officer a range and a reason to trust it. A single worst case number with no sensitivity is easy to attack and easy to dismiss.

Where the assumption decides the outcome for a receptor, say so in plain words in the summary. That is the sentence a case officer will quote.

Requesting access and documenting refusal

Do this properly, because the record matters at appeal.

Write to the occupier and, where different, the owner. Explain what the survey involves, how long it takes and what will be measured. Give a named contact and at least two date options. Follow up once in writing after a fortnight. Keep copies of everything, including the envelopes returned undelivered.

Do not offer payment for access and do not imply that refusal will count against the neighbour. Both come back at a hearing.

Where access is refused, the report should record the request and the refusal factually, without characterising the neighbour's motives. Inspectors read that section carefully and they notice tone.

How auditors test assumed layouts

A reviewing consultant appointed by the council or by an objector will do four things. They will look for the assumption schedule and check whether it is complete. They will pick two or three receptors and re-run them under different assumptions to see how sensitive the conclusion is. They will compare the assumed layouts against the council's own planning history for those addresses, which they can search as easily as you can. And they will check whether any window was excluded as non-habitable without justification.

Anticipate all four. The report that survives review is the one that already answers the questions the reviewer was going to ask.

Councils expect a suitably qualified author for daylight and sunlight work, and professional review is strongly advised. PlanScope drafts a Daylight and Sunlight Assessment and an Overshadowing Study structured to UK planning policy, with a "to be confirmed" flag wherever modelling inputs and survey results are missing, and reviewers in the professional directory can review and sign off a draft. 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.