Converting habitable room into a garage-Advice pls

A house we are looking at has a 18'x18' room that I would like to convert to a garage. It was built in '99, when they were given the choice at the planning stage they chose this room rather than a double garage.
What would be involved in doing this change? my main concern is the floor as it was built as a habitable room.
Any advice welcomed. Thanks.

It is likely that the existing floor will need hammering up as to afford you the necessary 100mm drop or to create fall to the garage door. This is to satisfy spillage reg's.

You will also need to fit a firedoor with 1/2 hour fire resistance and intumescent strips should you wish to maintain a door between habitable and the garage.

Finally, if the existing room (proposed garage) is integral, then you may need to insulate the ceiling or floor space if there is a habitable room above.

And put a garage door in the outside wall

Thanks noseall. The current floor is level with the rest of the house and sounds hollow underneath when tapped. Not sure what is going to be underneath, insulation? I am going to get a builder to have a look, just wanted to go in pre-armed with some info.
I have been trawling this excellent site and found plenty on the fire proofing and insulation side, every one seems to be doing it the opposite way round to me though i.e garage to habitable.
Looks as though it will be a bit more involved than just putting doors in...........doors!!!! I knew I had forgotten something