Q : My wife and I want to put a side extension on our house. Are you able to provide me a formula for working out overshadowing on September 22nd at 9am to 3pm?

A : This is a fair bit more specific than most of the questions we get here, but it’s another good one. It sounds to me as though someone has alerted you to one of the key objectives of ResCode; either that, or you have an uncommon interest in sun shading.

Among many other requirements, ResCode states that buildings should not “unreasonably overshadow existing secluded private open space”. The open space referred to is usually that of a neighbour, but open space on your own property can come under consideration if you live in a unit, for example, or on a small allotment.

It goes on to stipulate that at least 40 square metres of the space should be able to enjoy at least 5 hours of sunlight between 9am and 3pm on September 22nd (the equinox).

The logic is that if sunlight is available to an open area in September, then it will penetrate the space during summer when is higher in the sky. The extent to which it penetrates during the winter months depends on the configuration of the space, but ResCode is a step ahead. In September, the area lit by the sun must be at least 3 metres wide – this helps to ensure that even the longest winter shadows shouldn’t completely cover the area.

If the requirements sound complicated, try designing a building around them. It often requires careful balance of boundary setback, wall height and roof pitch that can generate some interesting forms.

Of course there are times when it can be impractical and even impossible to comply with the overshadowing guidelines, in which case an application can be made to Council for some concession. This can mean the preparation of a comprehensive submission that may require – and here’s the thing – consent from the neighbour.

As far as a formula is concerned, shadows are determined by location of a site, the time of year, the time of day and the height of a wall. In Melbourne on September 22nd the sun’s altitude is about 35 degrees above the horizon at 9am and again at 3pm. In the morning, the sun’s rays come from 59 degrees east of north and in afternoon they come from 59 degrees west of north.

Armed with this information, a drawing board and an adjustable set square you can plot the shadow cast by any building on the day. Alternatively, you can activate the appropriate command on your PC if you have access to computer-aided drafting (CAD) software.

But then again, you can always ask us.