Cumulative overshadowing from consented but unbuilt schemes

A cumulative daylight assessment that reports only existing conditions is incomplete on an urban site, and an experienced officer will say so. Where a neighbouring plot holds an unimplemented planning permission, the realistic future condition of the window you are assessing is the one that includes that consented building. Model it. Then report the proposal's own effect separately, because the thing that decides the application is the harm your scheme causes, not the harm the street will eventually suffer.

Getting that separation right is most of the skill in this work.

Why the baseline includes permissions that are not built

BRE 209 assesses the effect of a proposed development on the daylight and sunlight received by neighbouring windows and amenity spaces. The benchmarks are relative as well as absolute: a vertical sky component of 27 per cent at the window, and a retained value of at least 0.8 times the former value where the absolute benchmark is not met. Both of those depend on what "former" means.

If the plot next door has permission for a six-storey building and your scheme is assessed against the two-storey workshop still standing there, your figures overstate the loss attributable to your proposal. Equally, if you assess only against the consented scheme, you may understate the harm by crediting yourself with a reduction someone else has not yet caused.

The answer is to model both, and to say which one the conclusion rests on. Officers and inspectors are familiar with this and they are alert to an assessment that chooses the baseline which happens to produce the better result.

Local plan policy sometimes settles the point. Our note on local plan amenity policies and how they use BRE 209 sets out how authorities adopt or depart from the benchmarks, and some policies name the baseline directly.

Finding the relevant permissions

Start with the authority's planning register, searched by date range over the surrounding streets rather than by address, because a scheme that affects your assessment may sit two plots away and face a different road.

Six checks produce a defensible list.

1. All permissions granted within the last five years within the study area, taken from the register rather than from a summary list.
2. Any resolution to grant subject to a section 106 agreement, which is not yet a permission but is material.
3. Prior approval decisions under Part 20 and other Part 3 classes, which create additional storeys and are easy to miss because they are not described as full permissions.
4. Outline permissions with reserved matters outstanding, where only a maximum envelope is defined.
5. Appeal decisions allowing schemes the authority refused.
6. The published datasets on planning.data.gov.uk, as a cross-check on the register rather than a substitute for it.

Record, for each one, the reference, the decision date, the expiry date, whether any condition discharge applications have been made, and whether there is evidence of a start on site. That last column is what tells you whether the permission is still capable of being built.

Getting the approved drawings and modelling them

Approved drawings are usually on the register as PDFs at a reduced scale, and scaling from them introduces errors that compound in a three-dimensional model. Ask the authority for the drawings as issued, and where the file is large, ask for the specific approved elevations and floor plans by drawing number from the decision notice.

Where only a parameter plan exists, as with an outline permission, model the maximum envelope the parameters permit. That is the worst case the permission allows and it is the defensible assumption, provided you state it.

Three modelling points save arguments later.

• Model the consented building from the approved drawings alone, not from a planning application render or a marketing image, and cite the drawing numbers in the appendix.
• Set the consented building's finished floor levels from the approved sections, because an assumed ground level shifts every result.
• Keep the consented model as a separate layer that can be switched off, so the three scenarios are reproducible from one model rather than three.

An Overshadowing Study for amenity space works the same way: the sun-on-ground analysis has to run on the same scenario set as the window analysis, or the two halves of the report contradict each other.

Existing, future and proposed scenarios

Four scenarios rather than three, and the fourth is the one most assessments omit. It isolates the effect of your proposal against today's conditions, which is the comparison a neighbour will make and the one a committee understands.

Running all four costs little once the model exists. Reporting all four is what makes the assessment credible, because the reader can see that the conclusion does not depend on the choice of baseline.

Lapsed permissions and how to treat them

A full permission must normally be begun within three years under section 91 of the Town and Country Planning Act 1990, unless a different period is imposed. For an outline permission, section 92 requires an application for approval of reserved matters within three years and the development to begin within two years of the final approval.

A permission that has been lawfully begun by a material operation does not lapse, and it can be completed years later. So the question is not whether the permission is old, it is whether it has been implemented.

Three positions, with the treatment that holds up.

• Live permission with time remaining and no start on site: include in the future baseline.
• Lapsed permission never begun: exclude, and say in the report that you have excluded it and why.
• Permission begun by a material operation: include, and note the evidence of the start, which may be a condition discharge, a building control record or a site inspection.

The awkward case is a permission expiring within months of your decision date. Include it, and run a sensitivity test without it. An assessment that depends entirely on a permission about to lapse is one an inspector will discount.

Presenting the scenarios without losing the reader

Four scenarios across perhaps 200 windows produces a table nobody reads. The structure that works is a short narrative conclusion, then a summary table by property, then the full window-by-window results in an appendix.

In the summary, give for each affected property the number of windows assessed, the number meeting the benchmark in each scenario, and the range of retained values. Then state in one sentence which scenario the planning conclusion rests on and why.

Say plainly where a window fails. Reporting aggregate compliance percentages across a whole scheme hides the single flat that loses most, and that flat is the one the objector lives in. The evidence that carries weight on the other side of the counter is set out in our piece on objecting on daylight grounds, and it is a useful check before you decide how to present yours.

A Daylight and Sunlight Assessment also needs to name its software, its weather and sky model, and the date of the surrounding context survey. Without those, the results are not reproducible and an objector's consultant will produce different numbers.

Where cumulative effect changes the conclusion

The interesting case is a window already reduced below the absolute benchmark by a consented scheme. Assessed against the future baseline, your proposal's further reduction may satisfy the relative test comfortably, because the starting value is already low. Assessed against existing conditions, the combined reduction looks severe.

Both statements are true. The planning judgement is whether your scheme should bear the consequence of a reduction someone else has permission to cause, and that is argued rather than calculated.

What tends to persuade is honesty plus design response. Set out both figures, acknowledge that the choice of baseline changes the characterisation, and then show what the scheme does about it: a set-back at the upper floors facing the affected windows, a reduced depth to the rear wing, a lighter colour to a facing elevation to raise reflected light, or a relocated habitable room in your own scheme so that the harm falls on a circulation space.

An assessment that reports the favourable baseline only, and is then shown the other one at committee, loses the application and the officer's trust in everything else the submission says.

Enter your postcode on PlanScope to see which reports an urban infill scheme is likely to need. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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