Building under high set house?

Hi everyone.
I would appreciate if you investor guys help me on this. I have recently bought a 3 bdr high set house in Kippa Ring QLD (Moreton Bay Council). The house has 3 bdr kitchen lounge room etc up stair and same area with only bathroom down stair with legal hight. I bought thinking I can build a granny flat or dual living unit by spending approx. 25K area is 100 sqm. But as I never done this type of renovation I need help on this on following.
1) Can I build granny flat or dual living down stair as it got legal hight.
2) If we can what should I do for the approval? and approx. cost?
3) How much it cost to put tiles on floor ( 100 sqm)?, build two rooms outerwalls already there with windows, put kitchen, bath room already there, put new car port outside for two car?

4) After building granny flat downstair can we rent it out separately or we only can put one tenant?

If any body has done this type of work your advice would be great help for me. Thank you so much.

You cannot legally rent the downstairs out separately.

I don't know if that changes if you put in fire rated floors between upstairs and downstairs?