There is no guarantee that there is a DPM beneath the garage floor, or any insulation in the floor, and the outer walls are generally single skin and uninsulated. So I think you need to consider how warm you want the room to be.
For a warmer floor you can just lay a sheet of DPM (Visqueen) onto the concrete, lapped up the walls, lay 100mm PIR insulation on top of the DPM, then lay 18 or 22mm P5 T&G chipboard on top with the joints glued and pulled together with flooring straps (a so-called floating floor). Faster, warmer and cheaper than joisting then ply flooring
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