Location: essex, then chongqing, china and now essex again
a 90's built garaghe is likely to have been designed with the correct floor loadings to enable the later addition of another floor. thank the building regulations for that
as it's a 90's house (and therefore probably the usual 90's density if you're in an urban or suburban location) your most likely issues are likely to be related to overlooking and proximity to existing habitable rooms (ie being so keen to perve through your neighbouurs' windows that you're willing to build a whole new floor just for that sole purpose!)
can't answer the piers (pillars) question - although the issue is how the weight of the floor is transferred onto the slab - outrigged pads with steel uprights may be a more suitable solution strucutrally, but will be expensive.
brendan - i have a bedroom above my garage and it's the warmest room in the house. probably because it's the best insulated room in nthe house as it's the newest room in the house...plus my garage is heated so my little car doesn't get too cold (oh, and something to do with the beer fridge not working if the garage gets too cold). heat retention is a valid point though - i think you lose almost 25% of your heat through a poorly insulated floor (that's from memory, so could be completely wrong) - insulation is the key!
We did the same thing in a house built in 1990. Building control didn't make us check the foundations as they said that the original specs would have to have allowed for a second storey. All in, the materials were 5k and labour was 9k. We had to get the garage ceiling reboarded with a double skin of fire retardant plasterboard, then got the builders to put in 9" joists and do a full dept of insulation. Never had a problem with it being cold