infill slab - need to raise garage floor to house slab level

and insurance is also an issue if you have an unapproved habitable area in regular use (like a bedroom). If it is for a 'temporary' need such as a recreation area or kids playroom, you could do it by simply building a new floor on top of a timber frame over a waterproof membrane (builders plastic - bring it up the sides too by 200-300mm) directly onto the existing floor. Frame up using some 60x45 treated pine and cover with structafloor/ yellowtongue. Leave a gap around the edges of say25mm and use 70x35 framing for walls. Use sisalation style foil wrap facing the gap and then batts then plasterboard. Finished ceiling height should be 2.4m or more too although again for a temporary structure you could get away with 2.1. If you sell you will have an unapproved alteration - you can quickly remove it or tell the buyer it is being sold as is.

It is good to remember that although rules about construction of habitable and non-habitable parts of a residence the rules are less clear and even less enforced regarding your use of various parts of your residence. Like sheds and such like - if you do not upset neighbours and do not do anything outrageous then a variety of uses can be made of all sorts of areas - and are all the time. For example it is common for teenagers to move into a basement or a garage or even a shed - sometimes with some pretty major modifications. I know of others who have parked a caravan in their yard for a few years to allow some independence (for the parents & the offspring!). These are rarely a problem unless something dumb is done such as renting out or installing an illegal fireplace - or again the teenager upsets the neighbours so draws attention to the situation. This is what is called common sense approach.