As a rule of thumb, any structure over 9m on any one boundary has to be 1 .5m (1500mm) from that boundary. You may build right on the boundary any structure under 8.90m, making sure the gutter is in line with the boundary, but if there is an existing fence, our erectors have at least 400mm to work in. As a precaution, if you extend your garage at a later date to over 9m, you will need to be 1500mm from the boundary. Also double-check your sewer line and manhole. You are required to be 2m away from either, or sign an indemnity form from Council, allowing you to be closer.