extra bedrooms or 1bed flat

hi all i own a 3 bedroom house in lismore nsw with a 2bdm flat under and  room for a 1bdm flat most of the structural work has been done but i would need to create a laundry with hot watersystem in the two bedroom flat and add new toilet/bathroom/laundry/kitchen  to the new flat and rewire and new meter as all electics are from the existing flat as i pay the water cost plumbing should be cheap but i know it would be very much cheaper to just throw up a few walls and create a 5bedroom flat instead existing 2 bed flat has just had a new kitchen/floating floor carpets and paint and rents for 200/ week any ideas /comment or sugestions as to what would be the most practical way to increase the cashflow the most ie would a 5 bedder make more cash than a 1bedder(possibly 2) and the existing 2 bedder