Existing vs Proposed Shadow Diagrams: What Is the Difference?

Shadow Diagrams

When existing and proposed shadows appear on the same drawing set, the most important question is not simply which area is shaded. It is what has changed. Comparing the existing condition with the proposed development helps distinguish shadow that already occurs from the additional shadow associated with the proposal.

In Brief

An existing shadow condition represents the relevant site and built form before the proposed development, while a proposed shadow condition represents the site with the proposal included. Comparing the two helps identify the change in shadow associated with the development. The total proposed shadow should not automatically be interpreted as additional shadow, because some of that shadow may already occur under the existing condition.

What is the difference between existing and proposed Shadow Diagrams?

Existing and proposed shadow diagrams represent two different development conditions. The existing condition establishes the relevant shadow situation before the proposed development is introduced, while the proposed condition shows the shadow with the proposed design represented.

Looking at either condition in isolation can tell you where shadow falls at the nominated date and time. Comparing them provides another piece of information: how that shadow condition changes when the proposal is introduced.

This distinction matters because an area shown in shadow on the proposed diagram may already have been in shadow under the existing condition. The entire proposed shadow should therefore not automatically be treated as shadow created by the development.

The comparison between the two conditions helps separate the existing baseline from the change associated with the proposed built form.

What does an existing Shadow Diagram show?

An existing shadow diagram establishes the baseline condition used for comparison. It represents the relevant existing buildings, site conditions and other elements included within the scope of the shadow model.

The purpose of this baseline is to show where shadow already occurs before the proposed development is added. This is particularly useful where existing buildings already cast shadow across parts of the subject site or surrounding land.

An existing condition should not necessarily be interpreted as a model of every object that could cast any shadow in the real environment. The information represented should reflect the purpose and scope of the particular shadow assessment.

What matters for the comparison is that the relevant existing condition is established clearly enough to understand the change produced when the proposed development is introduced.

The proposed shadow is not automatically the additional shadow. Comparing existing and proposed conditions helps distinguish shadow that already occurs from the change associated with the development.

What does a proposed Shadow Diagram show?

A proposed shadow diagram represents the shadow condition with the proposed development included in the model. Depending on the project, this may involve a new building, additions to an existing building or other changes to the relevant built form.

The proposed condition shows the total modelled shadow for that scenario at the nominated date and time. Some of that shadow may result from existing buildings or site conditions, while another part may be associated with the proposed development.

For the comparison to be meaningful, the existing and proposed conditions should represent equivalent assessment conditions. The same date, time, site orientation and relevant context allow the development change to be separated from changes caused by comparing different moments or drawing conditions.

The reliability of that comparison also depends on the information represented in each model. For more on the relationship between project information and modelling reliability, see How Accurate Are Shadow Diagrams?

What is additional shadow?

Additional shadow is the change in shadow associated with the proposed development when the proposed condition is compared with the existing condition under equivalent assessment conditions.

Imagine, for example, that part of an adjoining property is already shaded by an existing building at the nominated assessment time. In the proposed condition, the same area remains shaded and the shadow extends further across the property. The existing shaded area is not automatically new shadow from the proposal; the change between the conditions is the relevant additional shadow.

This is why looking only at the proposed diagram can sometimes give an incomplete impression of what the development changes. A large shaded area may contain both existing shadow and additional shadow associated with the proposal.

Comparing the conditions allows the change to be identified without attributing every shaded area in the proposed scenario to the new development.

Why are existing and proposed shadows sometimes shown together?

Showing existing and proposed shadows together can make the difference between the two conditions easier to see. An overlay can allow the reader to identify areas that remain unchanged and areas where the proposed development alters the shadow extent.

Different tones, hatching, outlines or other graphic treatments may be used to distinguish the conditions. Some drawings may also identify additional shadow separately so that the change is immediately visible.

These conventions are not universal. Another drawing set may present the existing and proposed conditions on separate plans rather than combining them in one image.

The legend, title and annotations should therefore be used to establish what each graphic treatment represents. For a broader guide to interpreting these elements, see How to Read a Shadow Diagram.

Why should existing and proposed shadows use the same date and time?

A meaningful comparison needs to isolate the change caused by the development. Using equivalent assessment conditions helps achieve that.

If an existing shadow at one time of day were compared directly with a proposed shadow at another time, the difference between the drawings would reflect more than the proposed development. The position of the sun would also have changed.

The same principle applies to the assessment date. Comparing equivalent dates and times means the existing and proposed scenarios are being viewed under the same nominated solar condition.

This makes the before-and-after comparison easier to interpret because the proposed development, rather than a change in assessment conditions, becomes the principal difference between the two scenarios.

Does additional shadow mean the development fails?

No. The presence of additional shadow does not, by itself, determine whether a proposed development satisfies or fails a planning requirement.

The shadow comparison establishes what changes between the existing and proposed conditions. It can show where additional shadow occurs and how that change relates spatially to the subject site and surrounding context.

Whether that change is acceptable depends on the planning requirements relevant to the particular development. Those requirements may consider specific areas, periods of sunlight, existing conditions or other criteria that are separate from the geometric shadow comparison itself.

The diagrams provide evidence of the shadow change. Any assessment of solar access, overshadowing performance or planning compliance then needs to apply the controls and criteria relevant to the project.

What if there is already substantial existing shadow?

Where substantial shadow already exists, establishing the baseline becomes particularly important. Without the existing condition, it can be difficult to distinguish shadow associated with the proposal from shadow that would occur regardless of the development.

Existing shadow does not mean that any further change is automatically irrelevant. Equally, the presence of shadow in the proposed condition does not mean that all of it has been created by the proposal.

The purpose of the comparison is to avoid both assumptions. By establishing the baseline first, the drawings can show which areas remain unchanged and where the proposed development changes the shadow condition.

The significance of that change can then be considered separately against the planning requirements that apply to the development.

Can existing and proposed shadows appear on separate drawings?

Yes. Existing and proposed conditions do not need to appear as a single overlay in order to be compared.

Some drawing sets use separate diagrams for each condition, allowing the reader to compare equivalent views side by side. Others combine the conditions and use graphic treatments to distinguish existing, proposed or additional shadow.

Neither presentation should be interpreted purely by appearance. The drawing title, date, time, legend and annotations establish what condition is being represented and how the comparison should be read.

The important point is that the documentation allows the relevant conditions to be distinguished clearly enough for the change in shadow to be understood.

Existing and proposed conditions within the wider shadow assessment

Existing and proposed shadow diagrams serve different but connected purposes within the wider shadow assessment.

The existing condition establishes the baseline. The proposed condition establishes the modelled shadow with the development included. Comparing the two identifies the change associated with the proposal.

Keeping those steps separate prevents the total proposed shadow from being mistaken for the additional shadow created by the development. It also makes the documentation easier to interpret where substantial shadow already exists.

The shadow diagrams establish the geometric change. The wider planning assessment can then consider what that change means in relation to the requirements applying to the project.