The first place to start with a project like this is your local council. They can advise on the specific requirements and restrictions for turning a non-habitable space, such as a sunken garage, into a living area. In many cases, garages are not permitted to be used as habitable rooms without significant modifications because they are not constructed to the same standards as bedrooms or living spaces. This includes considerations around ceiling height, insulation, ventilation, fire safety, structural load, and damp proofing.
In terms of your specific questions, if you were to build a floating timber floor, the bearers and joists would typically be supported on piers designed for timber floors, with spacing and sizing determined by the load and span. A 140 x 45mm pine bearer could be used, but spacing, number of bearers, and whether intermediate joists are required would depend on the exact dimensions and the load expected. For sheeting, yellow tongue or red tongue flooring is common, but 3m x 800mm sheets may need cutting and staggering to fit the space properly. Underfloor ventilation is also a concern, especially in a sunken garage; in most cases, some form of ventilation or moisture barrier is required to prevent dampness, but exact requirements vary based on council regulations and building codes.
Because of all these factors, it is strongly recommended to use your council’s guidelines and obtain the appropriate building permits before proceeding. They will provide clear instructions on what modifications are allowed and how to safely and legally convert the space into a habitable room.
