It all depends on how the floor is constructed, if you have a load bearing wall in or near center of your house the size of the joists will be 150mm high and 2.4m is needed to get 2m headheight, if no loadbearing wall and the span is 5m then you will need 225mm joists and 2.5 is needed, if the span is more than 5m then a spanbreaker steel will be needed, this will sit above your existing ceiling joists and is normally a 200x200 uc so2.7 will be needed. These heights are minimun to generaly get 2m, the more height you have the better the conversion will be.