How long is a piece of string, how much noise and what type do you want to stop, what is below this room that determines how much sound you want to block.
Resilient battens and another layer of plasterboard on the ceiling below, insulation batts between the floor joists, resilient battens and a floating floor above.
Due to space restrictions and the construction I am guessing it will be hard to provide separation of the partition walls of the room above into the below structure, so any impact on those walls will allow sound transfer to occur easily to the below areas (or say speakers/TV mounted on that wall, the sound will get into the structure).
Not really my area of expertise, so just throwing out random thoughts.
As others said, you are playing games with planning issues, fire regulations especially have to be considered.