What is Overshadowing and what is a Shadow Diagram?

In Australia, often a requirement when proposing to build or extend a building, you will be required to provide a shadow diagram as a part of your planning or building permit submission. (this is not always required though)
What a Shadow Diagram drawing shows is how long a shadow will be cast from the proposed building or extension.
This shadow is referred to as ‘overshadowing’ when it is cast across neighbouring properties. The reason shadow diagrams are required is to ensure neighbouring properties adjacent to the property proposing the building work won’t be signifficantly impacted by the new building or extension. These drawings are reviewed by the planning departments at council, and sometimes building surveyors where a planning permit was not required.
The regulations and requirements between states and municipalites can vary with the information they would like to see in the form of shadow diagram drawings.
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In Victoria, the bare minimum required for shadow diagrams is to show how the shadows will be cast at 9am, 12pm and 3pm on September 22nd – the Equinox (Refer to the Victorian Building Regulations). Sometimes councils will ask for every hour from 9am to 3pm to be shown for September 22nd, and in some other cases, the council would also like shadow diagrams prepared for June 21st – the Winter Solstice. The reason some Victorian councils would like to see the Winter Soltice shadow diagrams is because this time of the year is when the shadows will be there very longest. This shows the maximum worst case scenario for overshadowing to neighbouring properties.

In New South Wales, it is different to Victoria in that they are generally mainly interested in seeing the June 21st – Winter soltice shadow diagrams – the worst case scenario. My thoughts that the reason for chossing June 21st over September 21st is due to the fact that Sydney is further up latitude than Melbourne, meaning the shadows will always be shorter than for the same height building in Melbourne.
However, some councils do also ask for additonal dates apart from June 21st, sometimes councils ask for September 22nd, and even 21st March – the other equinox.

What is being overshadowed?

What councils are looking for with the overshadowing is if private open spaces such as backyards are being overshadowed, light courts, solar panels on neighbouring properties and also sunlight being blocked from habitable room windows. (Habitable rooms are rooms such as bedrooms, living rooms, family rooms, dining rooms. Non-Habitable are rooms such as laundry, toilet, bathrooms)
The goal is for the design of the proposed building or extension to not severely impact on the neighbours due to the design of the building – whether it be it’s height or proximity to the boundary that may cause issue.

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Who prepare Shadow Diagrams?

Architects and building designers can prepare shadow diagrams. However, at times it is best to get a specialist to prepare the drawings for you to ensure accuracy of the shadow projections, as well as preparing additonal specialsed drawings to assist in demonstrating how shadows will be cast from the proposed building works for clarity during the planning permit process.
We can offer professional shadow diagram services to both building professionals and also the general public concerned about proposed building work adjacent to them. With over 12 years experience in shadow diagram preparation, you can be assured you will have the best documents required to assist you in your planning application or planning objection.