In addition to the DA requirements, the unit plan might be a problem. In our unit plan the garages, for those who have them, are on unit subsidiaries, not the unit area. That is, they are an extra bit of title linked to the main unit area. The terms of the lease (ACT is all leasehold) stipulate that each unit on the unit plan is for one dwelling and the subsidiaries are not be ‘dwelling places’. Perhaps that means they can’t be used to build another entire self-contained flat, or perhaps that means they can’t be another bedroom. I am not sure which.
Having said that, our set of town houses has one unit that has an approved extension that linked the unit to the garage and put a room that could be a bedroom into what had been the roof space of the garage. Another had a unique attached garage already and converted that to a bedroom and extra bathroom, also approved as far as I know.