building heights for habitable rooms must be atleast 2.4 non habitiable can be atleast 2.1 having said the benifit of 4x2 xany height below 3m is you shouldnt require any building permits as you are below 10sqm2 nor should you require a town planning permit but all area may have different bylaws. In the middle of one now part converversion. but can be easily converted into a granny flat