Building directly onto the boundary is not generally permitted; in terms of the National Building Regulations, you may not build a habitable room or structure on your property Boundary. Covered entertainment areas, lapas, garages, carports, lean-to shelters and temporary structures like shadeports etc. are usually allowed. However you will need your neighbour's consent and Council approval - dependent on the location, aesthetics of the structure and use thereof.  Bear in mind that you are not allowed to build closer than 3m away from any street boundary.  It's best to check with your local Council Building Services Department for their regulations for your particular area.