The first thing to consider is the amount of land that will be taken up with the new addition. Will the convenience of the extra building outweigh the loss of garden or lawn? Considering the aesthetic impact of the new extension is crucial. Making sure the new addition is sympathetic to the existing structure is important from a resale point of view. Once you have decided on a look and design for your new garage or granny flat you will need to engage the building services.
Local councils have different rules and regulations, and what is acceptable in a nearby suburb may not be allowable in your area, so check early in the project planning stage. The construction company you engage will be able to assist you in finding out if you need to have council approval. If you are a home builder you can contact your local council or a building surveyor to find out your legal requirements.
Check online for a list of local building services and building surveyors.