We also build oversized tiny houses as the added space to the width of the house helps to create a more functional space, its also just as easy to transport if you keep it under 3.5 metres wide
The legistration can change from state to state here in Australia but in NSW you could fall under a manufactured home or moveable dwelling which requires a section 68 approval
www.planningportal.nsw.gov.au/development-assessment/section-68-local-government-act-1993
You could also fall under the Granny flat/secondary dwelling registration here:
www.planningportal.nsw.gov.au/development-assessment/complying-development/granny-flats?fbclid=IwAR1S7wh79fDDWaYdJoGODv6YIQF-FxbhLDGntqVtL6F2nk_JiOi4F9HDW1c