I have a two story house in Brisbane, downstairs has been built in but contains no bedrooms, it consists of a kitchen/dining room, laundry, shower, pantry and a large living room. The height is 2.31 m and as such falls below the 2.4 m required for legal height for the living room component, however the legal height for the remainder is only 2.1 metres so well above. I have contacted and Architect to draft the changes and these will be submitted to a certifier who will hoperfully approve the new layout. The living room will however be submitted with an "alternative usage" form and will be called something else, not a living room.

The comments re removing plaster board are not entirely accurate as measurements are taken from the bottom of joists not the top, load bearing beams are not included so removing plaster you may only gain 20-25mm or so. Finding a certifier to do the work is tricky I called several and all of them said they were not interested in doing the work, one the told me they wanted about $2000 to submit the plans and the alternative usage form, I agreed to the cost and they since phoned back and said sorry we are to busy. The architect I am using has certifiers they use and is confident they will find one.  Hoper this helps, I will let you know how it washes up.