The mirror image baseline and existing obstructions

Choosing the daylight baseline existing obstruction scenario decides half the argument before any calculation runs. A daylight and sunlight assessment measures change, so it needs a starting condition, and on a site with a building about to be demolished there are at least three defensible starting conditions. Compare against the building as it stands and losses look modest. Compare against a cleared site and the same proposal appears to remove enormous amounts of light. Both sets of numbers are arithmetically correct. Only one of them answers the planning question.

Why the baseline choice matters more than the method

The tests in BRE 209 are comparative. Vertical sky component is assessed as a retained proportion of its former value, daylight distribution as a change in the area of a room receiving direct sky, and annual probable sunlight hours as a proportion of the existing figure.

Every one of those is a ratio. Change the denominator and the ratio moves, without a single window moving on the drawing.

That is why an assessment that buries its baseline in an appendix is doing the reader a disservice, and why a reviewer should look at the baseline before looking at a single result table. The same logic applies where existing conditions are unusually poor, a situation covered in assessing daylight where existing conditions are already poor.

Existing building against cleared site

The default position is to assess against the site as it exists on the date of the assessment, including any building standing on it.

That is the condition neighbours actually experience. A flat facing a four storey warehouse has never received the light it would receive if the warehouse vanished, so measuring the proposal against an imaginary open site overstates the loss and produces figures that misrepresent what anyone will notice.

The cleared site baseline has a narrower legitimate use. Where demolition has already happened, or where demolition is consented and would proceed regardless of the application, the site as cleared may fairly be the condition from which change should be measured. Even then the assessment should say what stood there, when it came down, and why the cleared condition is the right comparator.

Where a building is demolished purely to create a more favourable baseline, the assessment loses credibility. Officers and inspectors notice demolition timed shortly before an application.

The mirror image approach and its purpose

The mirror image test asks a different question: what would happen if the site were developed in the same form as the buildings around it.

The usual construction is to model a notional building on the application site that mirrors the height, depth and set-back of the property opposite, or of the neighbouring plot in the same terrace, and to assess the proposal against that. Where the proposal performs no worse than the mirrored form, the argument is that it does no more harm than the established pattern of the street already does.

It is most useful on urban infill sites where a gap in a terrace is being filled, or where a long-vacant plot sits between two developed ones. It is least useful where the surrounding pattern is itself irregular, because there is no coherent form to mirror.

Two cautions. A mirror image is an argument, not a baseline, and presenting it as though it were the existing condition invites a challenge. And the notional building has to be a realistic one: mirroring a form that could not be built on the application site because of its orientation or plot depth is not persuasive.

Vacant and derelict sites

Long vacant sites create an expectation problem. Neighbours have enjoyed daylight for years that the planning system never intended to protect, and they experience its removal as a loss whatever the assessment says.

The technically correct baseline is usually the site as it exists, including any derelict structure still standing. Where the structure is unsafe, part-demolished or skeletal, the assessment should describe what has been modelled and why, with photographs.

The planning argument runs alongside the technical one. An allocated site, or a site inside a settlement boundary with a development plan expectation of redevelopment, carries a reasonable expectation that something will be built. Saying so in the planning statement does more work than adjusting the model. It is the honest version of the point that an assessment cannot make on its own.

Consented but unbuilt schemes

Where the application site already has an unimplemented permission, that consented scheme is often the more meaningful comparator, because it represents what could lawfully be built tomorrow.

Assess it as an additional scenario rather than as a replacement for the existing condition. The strongest submissions present the existing situation as the primary baseline and the consented scheme as a fallback, then show that the proposal is no worse than the fallback.

Check three things before relying on a consent: that it is still extant, that it is capable of implementation, and that its drawings have been modelled accurately rather than approximated. A lapsed permission carries very little weight, and a consent that would require further approvals to build carries less than an applicant would like.

Presenting more than one scenario

Multiple scenarios are not hedging. They are how a reader is shown which factors drive the result.

A clear structure is to give the existing condition in full, then a short table comparing headline figures across the alternatives, then a statement of which scenario the applicant relies on and why. Keep the room schedule identical across scenarios so the only variable is the baseline.

State the date and source of the survey information used to build each model, and identify any property where the internal layout was assumed rather than known. Where the outcome flips between scenarios, say so plainly. An assessment that admits the sensitivity is more persuasive at committee than one that presents a single scenario as though no choice had been made.

The Daylight and Sunlight Assessment is often accompanied by sun-on-ground analysis where gardens and amenity space are affected, and the baseline question arises there too, since an Overshadowing Study measures shadow against an existing condition in exactly the same way.

Where baselines get challenged

Challenges arrive from three directions, and each has a standard answer.

Objectors argue that the existing building should be ignored because it will be demolished anyway, which produces the cleared site comparison and the largest apparent losses. The answer is that the relevant comparison is with the condition neighbours currently experience, supported by the date of the survey.

Local authority consultants argue the opposite where a proposal replaces a small building with a large one, saying the existing structure is so minor that the cleared site is the fair comparator. The answer depends on the facts and should be met with both scenarios rather than a refusal to model the alternative.

The third challenge is about transience. A neighbour may accept a reduction in annual sunlight hours but object to shadow at particular times of year, which is a different question from the baseline and is better addressed through the analysis discussed in transient overshadowing and permanent loss of sunlight. Neighbours preparing their own evidence face the same choices, as set out in daylight evidence in a neighbour objection.

Where a baseline is properly contested, the practical answer is to model both and let the decision maker choose. It costs a day of modelling and removes the strongest line of attack.

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, and daylight modelling is work councils expect from a suitably qualified assessor. We do not guarantee planning approval. To see which amenity documents a site is likely to need, enter your postcode on PlanScope.